Download Introduction to the Viruses

Introduction to the Viruses
In 1898, Friedrich Loeffler and Paul Frosch found evidence that the cause of foot-andmouth disease in livestock was an infectious particle smaller than any bacteria. This was
the first clue to the nature of viruses, genetic entities that lie somewhere in the grey area
between living and non-living states.
Viruses depend on the host cells that they infect to reproduce. When found outside of
host cells, viruses exist as a protein coat or capsid, sometimes enclosed within a
membrane. The capsid encloses either DNA or RNA which codes for the virus elements.
While in this form outside the cell, the virus is metabollically inert; examples of such
forms are pictured below.
Viral micrographs : To the left is an electron micrograph of a cluster of influenza viruses, each
about 100 nanometers (billionths of a meter) long; both membrane and protein coat are visible. On
the right is a micrograph of the virus that causes tobacco mosaic disease in tobacco plants.
When it comes into contact with a host cell, a virus can insert its genetic material into its host,
literally taking over the host's functions. An infected cell produces more viral protein and genetic
material instead of its usual products. Some viruses may remain dormant inside host cells for long
periods, causing no obvious change in their host cells (a stage known as the lysogenic phase). But
when a dormant virus is stimulated, it enters the lytic phase: new viruses are formed, self-assemble,
and burst out of the host cell, killing the cell and going on to infect other cells. The diagram below
at right shows a virus that attacks bacteria, known as the lambda bacteriophage, which measures
roughly 200 nanometers.
Viruses cause a number of diseases in eukaryotes. In humans, smallpox, the common cold,
chickenpox, influenza, shingles, herpes, polio, rabies, Ebola, hanta fever, and AIDS are examples
of viral diseases. Even some types of cancer -- though definitely not all -- have been linked to
viruses.
Viruses themselves have no fossil record, but it is quite possible that they have left traces in the
history of life. It has been hypothesized that viruses may be responsible for some of the extinctions
seen in the fossil record (Emiliani, 1993). It was once thought by some that outbreaks of viral
disease might have been responsible for mass extinctions, such as the extinction of the dinosaurs
and other life forms. This theory is hard to test but seems unlikely, since a given virus can
typically cause disease only in one species or in a group of related species. Even a hypothetical
virus that could infect and kill all dinosaurs, 65 million years ago, could not have infected the
ammonites or foraminifera that also went extinct at the same time.
On the other hand, because viruses can transfer genetic material between different species of host,
they are extensively used in genetic engineering. Viruses also carry out natural "genetic
engineering": a virus may incorporate some genetic material from its host as it is replicating, and
transfer this genetic information to a new host, even to a host unrelated to the previous host. This is
known as transduction, and in some cases it may serve as a means of evolutionary change -although it is not clear how important an evolutionary mechanism transduction actually is.
HIV
HIV stands for human immunodeficiency virus. It is part of a group of viruses known as
retroviruses and is the third one to be discovered since 1980. To be even more specific, it is a class
of retroviruses known as lentiviruses. Lentiviruses are traditionally associated with chronic arthritis
and anemia and cause slowly progressive and often fatal diseases. The earliest documented case of
a HIV infection was found in a 1959 vintage blood sample from central Africa.
The epidemic probably traveled down the Mombasa-Kinshasa highway in Africa and entered the
U.S. during the mid 1970s. The CDC began noticing in 1981 rare diseases in gay men which
traditionally affected people with damaged immune systems. They found diseases such as
pneumocystis, carinii pneumonia, and Kaposi's sarcoma.
The human immunodeficiency virus infects the immune system and can lead to AIDS. People can
be infected with the virus for many years without becoming ill and still transmit it to others.
The World Health Organization estimates that currently 17 million people are infected with HIV
and 4 million have AIDS worldwide. By the year 2000, the number could jump to 40 million
infected. More than 90% of AIDS occur in third world countries. The largest number of people
infected are in sub-Saharan Africa with over 2.5 million people infected with AIDS and more than
10 million adults with HIV.
In the U.S., AIDS is mostly found among homosexual/bisexual men. 47% of reported cases in
1993 were from this group. Drug users who use needles make up about 28% of the reported cases
in 1993. If you want to break this statistic down even further, you'll find that African American and
Hispanics make up 54% of the reported cases in 1993. Plus, this group also has the greatest annual
increase. Of heterosexuals who get HIV, 42% got it from contact with drug users. 78% of men and
74% of women who are heterosexual are African Americans or Hispanics too. Minorities account
for 51% of the reported cases among adults and adolescent males and 75% in males. Of people
who are less than 13 years old, 84% were minorities. In 1991, AIDS was the leading cause of death
for Black and Hispanic males 25-44 and the third leading cause for women.
There is good news in this epidemic. In a survey of American sexual habits in 1994, it was found
that Americans tend to have sex with partners of similar social standings. Thus it is unlikely to
spread out of the poor sector. It was also found that 80% of Americans only have sex with one
partner or non at all a year. However, HIV stands to be a major problem in third world countries
where a majority of the population is poor.
You know you are infected if you have HIV antibodies in your blood. The virus is easily killed by
alcohol and peroxide.
HIV travels from host to host via blood.
It can be passed during vaginal, , and oral intercourse. However, it is not believed that kissing
passes the disease. Before 1985, transfusions used by hemophiliacs were a major source of
infection. Needles used to inject drugs, tattooing, and ear piercing are also ways to transfer this
virus.
Pregnant women can pass it to the baby while it is still inside the uterus or by exposure to her blood
during childbirth. It can also be passed through breast milk.
It can't, however, penetrate the skin or be transmitted by blood sucking insects such as mosquitoes
and fleas or spread through the air, water or food.
Effects/ Symptoms
It doesn't produce any specific symptoms. It basically just weakens your immune system and can
lead to AIDS. AIDS stands for acquired immunodeficiency syndrome. (a syndrome is an
occurrence of a characteristic group or pattern of symptoms.) AIDS can appear anytime from two
to fifteen years to occur.
HIV constantly undergoes rapid genetic mutations in each of its victims that leaves the immune
system playing constant catch up. During the late stages of the infection, the body may lose and
replace two billion CD4 lymphocyte cells a day and new viruses appear at about 100 million to 680
million a day.
Possible Solutions/ Treatments/ Prevention
There are no cures, but treatments can help people live longer.
Since the virus mutates so quickly, it is very unlikely to be a vaccine available. However, 13
experimental AIDS vaccines have been given to 1,500 volunteers in the U.S. since 1988, though
federal health officials reject any type of large-scale trials due to lack of confidence in the vaccine.
The World Health Organization plans to test two vaccines in Thailand and Brazil in a desperate
attempt.
Prevention is still the best defense.
Influenza
Influenza is something most people get at least once and most of the time multiple times in their
lifetime. It can lead to pneumonia especially in the elderly, malnourished, and people with chronic
lung or heart problems.
Normally every year, about 25 to 50 million Americans catch the flu and between 10,000 to 40,000
die from it.
The virus has tons of genetic material protected by a sphere protected by layers of fat and protein.
All this genetic material is like having hundreds of pieces of legos which you can make tons of
different shapes with. Thus, it mutates easily and frequently. This means you'll probably never be
immune from the flu.
Influenza epidemics are probably as old as human history. It is hard to trace it's path because of it's
genetic symptoms.
It probably plagued Athens in 430 B.C. and destroyed Charlemagne's army in 876 A.D. There is
also strong evidence of an outbreak in the 16th century too. The first recorded pandemic was in
1580 across Africa and Europe. It killed thousands in 1647 as it moved from the Caribbean to New
England.
It is often known by colloquial names such as "la grippe," "jolly rant," or "the new acquaintance."
It was after the epidemic of 1732-33 in the American colonies that an English doctor named John
Huxham introduced an old Italian folk term which connected the colds, cough, fevers to the
astrological "influence" of the stars. Thus came the popularity of the term influenza. However, it
never really attracted much attention in history as other diseases stole the show – that is until the
pandemic of the "Spanish flue" in 1918-19.
This pandemic is 20th century's worst and deadliest faced by any modern western society. Contrary
to the name, the epidemic seemed to have started in the U.S. By conservative estimates, 21 million
people died worldwide out of a billion infected. At least 12 million died in India. There were about
550,000 deaths in the U.S. 1/10th of American workers were sick in bed during the winter of 1918.
1/5th of the U.S. Army and 28% of the civilians caught the flue. About 20,000 New Yorkers died
fall of that year. Inuit villages in Alaska were wiped out. Samoa lost 20% of its inhabitants.
However, no blood samples were saved from this epidemic so it will never be known exactly what
type of virus caused this epidemic. Epidemics like this could happen again. Influenza viruses such
as the Hong Kong flu, which killed about 70,000 people in the U.S. in 1968-9, remind us that
influenza should not be completely ignored.
The flu virus was first isolated from chickens in 1901, but it was recognized as so until 1955. The
flu virus was also isolated from runny pig snouts in 1931. This virus became know as "swine flu"
There are three main types of influenza simply named A, B, C. Influenza A is the most common
was discovered in 1933. Influenza B was discovered in 1940 and influenza C in 1947. Variants of
these basic types are named after the place it first strikes. For example, there is the Hong Kong (B),
New Jersey (A), and Bangkok (A). Influenza A has also undergone two antigenic shifts since 1918
in 1957 and 1968.
In 1976, Ford signed a $135 million immunization campaign against the swine flu in fear that the
1918 epidemic was back. In the end, it reached only a quarter of the U.S. population. However, it
also happened that Guillain-Barri syndrome, which is a type of rare paralysis, appeared ten times as
much in vaccines. The government eventually paid out another $93 million in damages, putting a
permanent crimp in American pharmaceutical industry's eagerness for making any vaccines.
Virions are usually roughly spherical and about 200nm in diameter. The envelope contains rigid
"spikes" of haemagglutinin and neuraminidase which form a characteristic halo of projections
around negatively stained virus particles.
Influenza not only affects humans but they also infect other animals such as pigs and turkey. Not
only that, but it easily crosses species. Thus, new viruses are probably created in pigs and water
fowl like ducks and seagulls and then later passed on to other creatures. For example, in 1980, the
virus produced an epidemic in seals and caused conjunctivitis in humans who tried to help.
The influenza virus can also last for hours in dried mucus.
Generic symptoms such as fever, fatigue, coughing, and aches.
Hepatitis
The word hepatitis means inflammation of the liver. It can be caused by a variety of organisms,
drugs, exposure to toxic chemicals, and alcohol. However, viral hepatitis is the most common.
The viruses that cause hepatitis was first identified in the 1960s – 70s with the help of a study of
children at the Willowbrook State School for the mentally Handicapped in Long Island, New York.
Filthy living conditions in the school created an infection rate of almost 100%.
Viruses that cause hepatitis are named A, B, C, delta, and E. The hepatitis A virus (also called
HAV) and the hepatitis B virus (also called HBV) are the most common.
The HAV closely resembles the poliovirus. It consists of a single strand of RNA enclosed in an
icosahedral (20 sided) shell. It often shows up where sanitation and hygiene is poor. However, as
with some viruses, early infection, which happen in poor countries, mean mild symptoms.
Outbreaks of HAV in the U.S. have mostly occurred in day care centers, among gay men, and in
connection with polluted water or uncooked food. For example, shellfish from contaminated water.
It has been on a decline since 1971 when approximately 50,000 Americans were infected, mostly
in the 15-24 age group. HAV also shows strong "socioeconomic" ties, or how rich you are in
society. HAV infects both humans and primates. People are immune to the disease once they've
caught it once.
The HBV however, infects only humans, even though they have close relatives which infect ducks,
woodchucks, and squirrels. HBV can stay dangerous on needles, surgical tools, thorns, and sharp
stones. In Asia and Africa, HBV is commonly passed during childbirth or breast milk. It is also
ranked as the world's most common virus to cause cancer since it often brings with it liver cancer.
Some people (5% to 10%) who are exposed to HBV will not become seriously ill, but will still be
capable of infecting others. There is an estimated 400,000 to 800,000 carriers in the U.S. and 150
million to 200 million carriers worldwide. HBV has been on a decline too. It fell 59% between
1985 and 1993. A vaccine for HBV was developed in 1982.
Hepatitis C used to be called non A, non B before it was given the C. It is related to the yellow
fever virus and is also the leading cause of chronic liver disease and cirrhosis. The good news is
that this virus has been on a decline as well. It fell by over 50% in the U.S. between 1988 and
1993.
Hepatitis D or delta can only live in cells that are infected with HBV. In a sense, you are getting a
double infection. It borrows HBV's coating material and boosts the severity of the disease.
HAV is acquired by fecal contamination or through oral/ activities.
HBV is transmitted by infected though contact of blood and sexually through saliva, semen, or
menstrual blood. About 70% of children who's mothers have HBV will be HBV positive by they
time they are six months old.
Hepatitis C is caused by a variety of viruses. They are all blood born and can be spread by blood
transfusion. They can also be transmitted sexually.
Hepatitis E travels via fecal-oral contact and contaminated water, much like HAV. It has caused
major epidemic in Asia, Africa, and Mexico.
Effects/ Symptoms
For the HAV, symptoms usually appear 2 to 6 weeks after exposure. The virus is most contagious
during this period. The liver becomes tender and swollen. Bilirubin, which is a substance produced
by the liver to break down old blood cells, accumulate in the blood stream. Victims experience
jaundice, weakness, brownish urine, lack of appetite, and low-grade fever for several days.
HBV and hepatitis C have the same symptoms as HAV except symptoms show up between six
weeks to six months after infection. Symptoms usually last longer and more severe however. About
10% of the people in the U.S. with HBV develop chronic liver disease. About 50% of the people in
the U.S. with hepatitis C develop this disease.
Hepatitis E virus (HEV) is a spherical, nonenveloped, single stranded RNA virus that is
approximately 32 to 34 nm in diameter.
There is no specific treatment for HAV. Good hygiene and cautious behavior can lower the risk.
There is no vaccine, but a shot of gamma globulin can reduce the risk of contracting the virus for
up to six months. The gamma globlin can also prevent symptoms if administered immediately after
exposure.
There is a vaccine for the HBV available.
Bed rest, a healthy diet, and restricted alcohol use is the best treatment for hepatitis C. There is
usually recovery in four months.
You can prevent being infected by practicing safe sex, not sharing needles, avoid contact with
blood or other body secretions, etc.
Hepatitis A Virus
Smallpox
This is the first disease to be completely eradicated from nature. It is also the least understood and
most destructive disease in history. It is part of the orthopoxvirus genus which include cowpox,
rabbitpox, monkeypox, amelpox, but not chickenpox (which is caused by the varicella virus)
One droplet of exhaled moisture from an infected person contains a thousand more viruses than
needed to infect someone.
The first recorded attack was in ancient Egypt. The last recorded attack was in 1977. There was
possible signs of smallpox rash on the mummy of Rameses V who died in 1156 B.C. The Roman
Empire lost over 1/3 of their subjects in certain areas during a 15-year epidemic beginning 165
A.D. It decimated the indigenous population in America. Pocahontas died from it in 1617 during
her trip to London. It killed Queen Marry II of England in 1694, Emperor Joseph I of Austria in
1711, King Louis XV of France in 1774, Emperor Gokwomyo of Japan in 1654, and Emperor
Komei of Japan in 1867. George Washington managed to survive an attack after visiting Barbados
in 1751, but it left him scarred. 1/3 of the population of Iceland died in 1707.
The term "smallpox" was first used in Englihs in the 16th century as a translation of the French
term la petite vérole (vérole means pox).
1714, Greek physician Emanuel Timoni published an article about preventing the disease by
embedding a knife into a victim's rash and then scratching it on a healthy person. This was similar
to many folk techniques known in China, India, and Western Asia. After Lady Mary Wortley
Montagu, wife of an English ambassador to Turkey, in 1721 allowed Gypsy women to immunize
her infant son, it became popular to do so in royal families. Of course, the king first tested it on six
condemned prisoners and 11 children.
In 1721, Cotton Mather heard about the practice from his slave and convinced other
Congregational ministers to use the method since smallpox threatened to depopulate Boston. 6,000
residents got the disease and 900 died. Only 287 were eventually immunized of which only six
died. This success was not enough to convince most people however. It was still illegal in
American colonies. People were afraid it could start epidemics. Plus, there was no explanation was
to why it should work. The process was counter intuitive and as a result, the public remained
skeptical. Ben Franklin couldn't believe this and as a result, his four-year-old son died in the 1736
epidemic. However, John Quincy Adams was inoculated at eight in 1775. Mozart also had
smallpox. Abraham Lincoln fell ill to smallpox hours after the gettysburg address. Catherine the
Great of Russia, with some pressure from Voltaire , paid an English doctor 10,000 pounds and 500
pounds a year to inoculate the royal court. George Washington ordered his army to be inoculated
during the revolutionary war. After that, the anti-inoculation laws were revoked.
In 1796, Edward Jenner, an English country doctor found a less threatening procedure to vaccinate
someone. He injected an eight-year-old boy named James Philipps with a fluid or "vaccinia" from a
cowpox pustule. At first, the Royal Society rejected Jenner's paper on this type of inoculation.
However, soon after publishing a book which provoked wide spread testing confirming the
safeness of this method, it was won approval. Parliament awarded Jenner with $150,000 at early
19th century prices.
By 1950, North America, Central America, and Europe were free from the disease. However, there
was still ten to twenty million cases a year and two million deaths. It used to be so common in
Bangladesh that the word from springtime, bashunto, was also used to refer to smallpox.
A $330 million, ten year campaign first proposed by the Soviet Union in 1958 was started in 1967
to eradicated smallpox was begun. In 1975 the last incidence of wild variola major, which is the
most virulent form, was cured in the three-year-old Bangladeshi girl named Rahima Banu. In 1977,
the last natural occurrence of smallpox caused by variola minor occurred in a hospital worker
named Ali Maow Maalin.
The germ is now stored in liquid nitrogen in about 600 one inch long test tubes in max security in
the CDC in Atlanta and at the Institute for Viral Preparations in Moscow. The U.S. still stores 17
million doses of vaccine and military personnel are still vaccinated. Occasionally they catch it from
the vaccination and would spread it to others. Researched have mapped its DNA sequence for
future reference. There is debate as to whether to keep the last remaining samples of the disease or
to destroy it in a pressure cooker at 248 degrees F for two 45-minute cycles.
People are so afraid of this disease that Russian scientists even dug up victims buried in Siberia to
see if the virus was present. It wasn't.
Possible Solutions/ Treatments/ Prevention
There is a vaccine.
Herpes
There are over 90 members in this family that harass everything from humans to fungi. They have
probably been around for eons since they can be found everywhere, even isolated primitive tribes.
The word "herpes" comes from Greek and it means "to creep." The term has been used for
thousands of years. There are basically two types of herpes that infect humans and are referred to
as herpes simplex virus 1 (HSV-1) and herpes simplex virus 2 (HSV-2).
HSV-1 was first described by a Roman doctor named Herodotus around 100 A.D. He described it
as a "herpetic eruptions which appear about the mouth at the crisis of simple fevers." HSV-2 was
publicly reported 1,600 years later by a Frenchman named Astruc. These two viruses are very
closely related. Half their genetic codes are identical.
This disease affects about 30 million Americans. There is about 500,000 new cases each year.
Symptoms can be triggered by fever, other viral infections, fatigue, stress, menstruation, ultraviolet
energy from the sun, or even the wind.
It is transmitted by direct contact with an active sore or a genital secretion containing viruses.
Pregnant women can also infect babies during childbirth. The disease can be transmitted by people
who show no symptoms.
Effects/ Symptoms
Symptoms occur two to ten days after exposure, and generally lasts for two to three weeks. The
initial outbreak is the worst.
HSV-1 causes cold sores around the mouth. HSV-2 causes the same thing but around the genitals.
The virus produces flu-like symptoms such as muscle aches, swollen glands, fever, and sometimes
shooting pain in the legs and abdomen. Symptoms will subside without treatment.
Possible Solutions/ Treatments/ Prevention
Once it is inside the body, it's impossible to eradicate. They can survive an entire lifetime in the
host, hiding in the nerve endings where the immune system can find it. Sometimes it produces
symptoms, but often not..
Some antiviral drugs such as acyclovir can help relieve the severity of the symptoms by reducing
the number of outbreaks and the duration of the attacks.
Yellow Fever
Viruses can shape human history, as did the yellow fever virus. It shaped the development of
America. In 1800, Thomas Jefferson stated that "yellow fever will discourage the growth of great
cities in our nation."
This fear drove the need for basic public health services such as clean water supply, sewage
systems, and a health department.
Epidemics in Barbados in 1647, Yucatan and Cuba in 1648 signaled the establishment of this
disease in the Western Hemisphere. Since the black slaves in the Caribbean are immune to the
disease, it suggests that it originally came from Africa.
Previously unexposed people had a 50% chance of survival.
It struck Charleston and Philadelphia for the first time in 1699. New York lost 10% of it's
population in 1702. In 1793, Philadelphia lost 15% of it's population when over 4,000 died between
August and October. Those who didn't die tried to flee the city. Society was ripped apart. Family
members abandoned each other. Natives blamed foreigners, which were Germans and Haitian
immigrants this time. Clerics found reason not to assist the helpless, and a provisional government
was established to maintain some city services in the absence of office holders. It was much like it
was during the Black Plague. It was also one reason Philadelphia became less of a shipping/trading
port than New York.
In 1802, Napoleon tried to defeat a slave revolt in Haiti. Unfortunately his troops met up with the
yellow fever which killed 22,000 of his 25,000 troops. It made him decide to sell the Louisiana
Territory to the U.S. in 1803 thus doubling the size of the U.S. overnight. Haiti also declared
independence soon after in 1804 to become the first black nation to be free from European colonial
rule.
The mosquito was first identified as a carrier in 1881 and later proved to be so by Dr. Walter
Reed's Yellow Fever Commission and from recent proof that mosquitoes caused malaria. in 1902.
However, many people have guessed at this. A Cuban physician named Doctor Carlos Finlay
guessed it was Aedes in 1818.
A vaccine was developed in 1937 and was first given out on a mass basis in French West Africa in
1939. An epidemic in Ethiopia in 1960-2 infected over 100,000 and killed 1/3 of those infected.
The mortality rate for yellow fever can be as high as 10% in large outbreaks but is usually less. It
currently occurs in underdeveloped rural areas with lax control measures and /or immunization. In
south America there is over 10,000 deaths a year. There was also an outbreak in Kenya in 1993.
Causes/ Source
A female Aedes mosquito transmits this disease, which is not native to Americas. Because the A.
aegypti lays its eggs in containers of clean water, they thrive around human settlements. They
especially like humid conditions and places where the average temperature is above 72 degrees F.
A variety of this disease can pass between monkeys and humans via a number of forest canopy
mosquitoes in Africa and South America. Thus, it is impossible to wipe out this disease.
Effects/ Symptoms
Symptoms include yellow skin and black vomit.
Possible Solutions/ Treatments/ Prevention
A vaccine is available.
What The Heck is a Virus?
~~~~~
Introduction
First, let me tell you what a virus is NOT. A virus is not a bacterium, nor an independently-living
organism. A virus cannot survive in the absence of a living cell within which to synthesize copies
of itself (replicate). Antibiotics do not harm a virus; it is for this reason that treatment for the "flu"
for example, is mainly to help ease the symptoms of the illness rather than to kill the organism
which causes the "flu" (Influenza virus Please see: What the Heck is the "Flu"?).
Well then, what IS it?
Now, is there a simple explanation which can define what a virus IS? Hmmmm... that's actually a
tough question. A virus is not strictly alive.. nor is it strictly dead... A virus has some fundamental
information (genes made of DNA or RNA) which allows it to make copies of itself. However, the
virus must be inside a living cell of some kind before the information can be used. In fact, the
information won't be made available unless the virus enters a living cell. It is this entrance of a
virus into a cell which is called a viral infection. Too, the virus is very, very small relative to the
size of a living cell. Therefore, the information the virus can carry is actually not enough to allow it
to make copies (replicate). The virus uses the cell's machinery and some of the cell's enzymes to
generate virus parts which are later assembled into thousands of new, mature, infectious virus
which can leave the cell to infect other cells. Poliomyelitis virus for example, may have over one
million copies of its basic genetic information (RNA) inside a single, infected human intestinal
mucosal cell.
What does a virus look like?
Moving from the outside to the inside, here are some parts of a virus which are common to many
different kinds of viruses: capsid, core, genetic material (DNA or RNA). The capsid is the outer
shell of the virus which encloses the genetic material within. The capsid is actually made of many,
many identical individual proteins which are assembled very precisely to form the capsid structure.
Sometimes there will be a protein core underneath the capsid which also surrounds the genetic
material. Some viruses may have an additional covering on the outside called an envelope. An
envelope is kind of like skin around the outside of the virus. The envelope is actually a lipid bilayer
(membrane) with proteins embedded within the membrane. If you examine a baseball, take it apart,
you will see how some viruses are assembled. The cover of the baseball (envelope), the tightlywoven thread (capsid), and the rubber core (genetic material) can be used to represent the parts of
some viruses.
What do viruses actually do?
All viruses only exist and make more viruses. And with the possible exception of bacterial viruses
which kill harmful bacteria, all viruses appear to be harmful because their replication leads to the
death of the cell which the virus entered. A virus enters a cell by first attaching to a specific
structure on the cell's surface via a specific structure on the virus surface. Depending on the virus,
either the entire virus enters the cell, or perhaps only the genetic material of the virus is injected
into the cell. In either case however, the ultimate result of viral infection is the exposure of virus
genetic material inside the entered cell. Then, the virus material essentially "takes over" the cell
and nothing but viral parts are made, which assemble into many complete viruses. These viruses
are mature and leave the cell either by a process called "budding" (just one or a few viruses at a
time leave the cell) or by a process called lysis (the cellular membrane ruptures and releases all of
the virus particles at once).
What things can become infected by a virus
So far, there is not a living thing identified that doesn't have some sort of susceptibility to a
particular virus. Plants, animals, bacteria - every living thing, whether multicellular or singlecelled, can be infected with a virus specific for the organism. And, within a species, there may be
100 or more different viruses which can infect that species alone. So, whenever viruses are
discussed, they are discussed as being either plant, animal or bacterial viruses - which means that
an animal virus only infects a certain animal, and a plant virus only infects a particular plant. We
say that a virus is specific for a particular thing if the virus infects only that thing. So, there are
viruses which infect only humans (smallpox), some which infect humans and one or two additional
kinds of animals (influenza), some which infect only a particular kind of plant (tobacco mosaic
virus), and some which infect only a particular species of bacteria (lambda bacteriophage which
infects E. coli).
How many kinds of viruses are there?
Viruses come in all shapes and sizes and have an enormous array of different kinds of organization
of basic genetic material within them. And, it is the arrangement and type of genetic material which
is the method used for sub-classification of a given group of viruses. For example, the animal virus
group can be sub-divided into the following sub-groups: double-stranded DNA; single-stranded
DNA; double-stranded RNA; single-stranded RNA, and, retroviruses (a very unique kind of singlestranded RNA virus). An example of a human double-stranded DNA virus is Epstein-Barr virus
(EBV). An example of a human single-stranded RNA virus is Influenza virus, Type A). An
example of a human retrovirus is Human Immunodeficiency virus (HIV). As you can see, unlike
humans, the arrangement and kind of genetic material within viruses can be very diverse. However,
for all viruses, regardless of the kind or arrangement of genetic material, the virus is capable of
replicating within a living cell and can produce progeny (offspring) which are usually absolutely
identical to the original virus. You may wish to take a look at David Sander's information about
viruses. Please see: David Sander's Complete Virology Information, Tulane
Do viruses change
Sometimes during the process of viral replication, mutations occur. If the mutation is harmful, the
new virus particle might no longer be functional (infectious). However, because a given virus can
generate many,. many copies of itself, even if 200,000 particles are no good, 100 might still be just
fine. Further, some mutations don't lead to harm to the virus, but instead lead to a functional but
now brand-new strain of virus (Influenza virus can do this; consequently, there are several different
strains of this virus which have to be identified each year in order to make a vaccine against the
particular strain which might cause the "flu").
What protects things against viral infection?
Humans are protected in a couple of ways. First, if a particular virus infects one or more cells of a
given tissue in our body, the infection leads to the synthesis and secretion of substances called
interferons. Interferons are proteins and may be designated as alpha, beta, or gamma interferon).
These proteins interact with adjacent cells which help adjacent cells become more resistant to
infection by the virus. Sometimes, this resistance isn't quite good enough to prevent the spread of
the virus to more and more cells, and we begin to feel sick (we are now experiencing a disease
caused by the viral infection). Now however, the body's immune system takes over and begins to
fight the infection by killing the virus on the outside of the cells, and kills the infected cells, too.
The killing of the infected cells prevents the spread of the virus, since as was stated above, a virus
requires a living cell in order for the virus to be able to replicate. Eventually, the virus will be
completely removed, and we'll get over the illness. HIV is an exception to this situation because
HIV infects cells of the immune system which are necessary to kill the infected cells. So, although
HIV does not itself directly cause the condition known as AIDS, the eventual death of immune
cells due to infection with HIV allows other infections to harm a person.
Recently, there have been agents designed in the laboratory and isolated from natural sources
which are being used to fight certain viral infections. These agents are not called antibiotics
however, since they are effective only for viruses and have not been isolated from other organisms
capable of killing a virus. So far, no agents have been identified which are secreted by a cell which
actually kills a virus. You may be familiar with the drug called Acyclovir which is used to inhibit
the replication of Herpesvirus; and, AZT and HIV protease inhibitors which are used to inhibit the
replication of HIV.
Plants are protected from certain viruses by substances which coat leaves and stems and "closingoff" systems which generate a walled-off area within the plant at the source of the infection.
Bacteria can be protected from bacterial-specific viruses through the action of enzymes inside the
bacterium's cell. However, if a bacterial virus (called a bacteriophage) infects one cell, usually
within a very short time, all of the bacterial cells will be killed. If there are no other bacterial cells
of that particular species around for that particular virus, however, the virus will die, too.
Case #6: Virus Or Bacterium?
Our Challenge: Determine the ways that viruses and bacteria are different.
Because bacteria and viruses cause many of the diseases we're familiar with, people often confuse
these two microbes. But viruses are as different from bacteria as goldfish are from giraffes.
For one thing, they differ greatly in size. The biggest viruses are only as large as the tiniest
bacteria.
Another difference is their structure. Bacteria are complex
compared to viruses.
A typical bacterium has a rigid cell wall and a thin, rubbery cell
membrane surrounding the fluid, or cytoplasm (sigh-toe-plasm),
inside the cell. A bacterium contains all of the genetic information
needed to make copies of itself—its DNA—in a structure called a
chromosome (crow-moe-soam). In addition, it may have extra
loose bits of DNA called plasmids floating in the cytoplasm.
Bacteria also have ribosomes (rye-bo-soams), tools necessary for
copying DNA so bacteria can reproduce. Some have threadlike
structures called flagella that they use to move.
A virus may or may not have an outermost spiky layer called the
envelope. All viruses have a protein coat and a core of genetic
material, either DNA or RNA. And that's it. Period.
© Eric MacDicken
Which brings us to the main difference between viruses and
bacteria—the way they reproduce.
© Eric MacDicken
Viral vs. Bacterial Reproduction
Bacteria contain the genetic blueprint (DNA) and all the tools (ribosomes, proteins, etc.) they need
to reproduce themselves.
Viruses are moochers. They contain only a limited genetic blueprint and they don't have the
necessary building tools. They have to invade other cells and hijack their cellular machinery to
reproduce. Viruses invade by attaching to a cell and injecting their genes or by being swallowed up
by the cell.
Here's an example of viral infection. This is virus version of the horror movie Alien.
These are T4 bacteriophages (back-tear-e-oh-faj-es). They
are a kind of virus that infects bacteria. Here they are
landing on the surface of an E. coli bacterium.
© James A. Sullivan
© James A.
Sullivan
The bacteriophage cuts a hole in the E. coli's cell wall. It then injects its genetic material into the
bacterium. By taking over the E. coli's genetic machinery, the viral genes tell the bacterium to
begin making new virus parts. These parts come together to make whole new viruses inside the
bacterium.
Eventually so many new viruses are made that the E. coli bursts open and dies, releasing all those
new viruses to infect more cells!
© James A. Sullivan
Case #4: Friend Or Foe?
Our Challenge: Some people think it would be great if scientists could wipe out all the microbial
bugs! Should we do it, and why or why not?
As a microbe sleuth, I get a lot of requests to track down the nasty bugs that are making people
sick. It's true that some microbes cause health problems such as strep throat, chickenpox and the
common cold. Unfortunately, because a small number of microbes cause disease, all microbes have
got a bad rap.
It seems only a few bad microbes get almost all the press. You just don't hear as much about all the
many GOOD things microbes do every day.
So I've created my own "most wanted" gallery, except instead of thugs, my gallery contains
pictures of some microbe heroes who make our lives better.
© Eric MacDicken
Lactobacillus acidophilus (lack-toe-bah-sill-us
acid-off-ill-us): one of the bacteria gang wanted
for turning milk into yogurt.
© Eric MacDicken
Streptomyces (strep-toe-my-seas): soil bacteria
wanted for making streptomycin, an antibiotic
used to treat infections.
© Eric MacDicken
Pseudomonas putida (sue-doe-moan-us pootea-dah): one of many microbes wanted for
cleaning wastes from sewage water at water
treatment plants.
© Eric MacDicken
Escherichia coli (Esh-er-ish-e-ah coal-eye):
one of many kinds of microbes that live in your
gut. Wanted for helping you digest your food
every day.
© Eric MacDicken
Bacillus thuringiensis (bah-sill-us ther-in-gee-in-sis): a.k.a "Bt", a
common soil bacterium. Wanted as a natural pest-killer in gardens
and on crops.
There are many other important jobs microbes do. They are used to make medicine. They break
down the oil from oil spills. They make about half of the oxygen we breathe. They are the
foundation of the food chain that feeds all life on earth.
In short, microbes are much more our friends than our enemies.
Case #5: Working For Us
Our Challenge: What are some of the microbe-made things we use everyday?
We've been using microbes for thousands of years to make products we
need and enjoy. For example, you can thank fungi for the cheese on your
cheeseburger and yeast for your bun. Cheese and bread are two microbemade foods people have been enjoying since civilization began, only
unlike our ancient ancestors, WE know that microbes are what makes
bread rise and milk curdle into cheese.
© Eric MacDicken
Over the past 50 years, we've begun harnessing microbes to do all kinds
of new work for us. Here are some examples of microbes at work in pollution control, medicine
and industry.
Microbial Janitors
Researchers are using bacteria that eat methane
gas to clean up hazardous waste dumps and
landfills. These methane-munching bacteria, or
methanotrophs, (meth-an-oh-trofs) make an
enzyme that can break down more than 250 nasty
pollutants into harmless molecules. By piping
methane into the soil, we can increase growth of
the methanotrophs that normally live in the
polluted soil. More methanotrophs means faster
pollution break up.
We're using bacteria, like those pictured here, as one of the tools to clean
up oil spills, like the Exxon Valdez mess. These bacteria chow on the oil,
turning it into carbon dioxide and other harmless by-products.
© Michael Abbey
Visuals Unlimited
Microbes Make Medicine
Fungi and bacteria produce powerful antibiotics such as penicillin (penih-sill-en) and tetracycline (teh-truh-sigh-klin). These are drugs we use
to fight off nasty bacteria that cause sore throats, ear infections, diarrhea
and other discomforts.
Penicillin mold
© Dennis Kunkel
Scientists have changed, or engineered, the genetic blueprints of bacteria and yeasts to turn them
into mini medicine-making factories. They stick genes for medicines they want to make—say,
insulin for diabetes—into the microbial cells, as if adding new building information to the
microbe's blueprint. The scientists then grow the microbes in huge containers called fermenters
where they happily reproduce into billions, all making insulin.
Industrial-Strength Microbes
Bacteria
Industrial fermenters with a person for
scale. What you see is only the very top of
one of these large fermentors.
Courtesy of Novo Nordisk
Bacteria consist of only a single cell, but don't let their small
size and seeming simplicity fool you. They're an amazingly
complex and fascinating group of creatures. Bacteria have
been found that can live in temperatures above the boiling
point and in cold that would freeze your blood. They "eat"
everything from sugar and starch to sunlight, sulfur and iron.
There's even a species of bacteria—Deinococcus
radiodurans—that can withstand blasts of radiation 1,000
times greater than would kill a human being.
Classification
Borrelia burgdorferi
Bacteria fall into a category of life called
Nelson,
ASM
MicrobeLibrary
the Prokaryotes (pro-carry-oats).
Prokaryotes' genetic material, or DNA, is not enclosed in a cellular
compartment called the nucleus.
Bacteria and archaea are the only prokaryotes. All other life forms are
Eukaryotes (you-carry-oats), creatures whose cells have nuclei.
Leucothrix mucor (Note: viruses are not considered true cells, so they don't fit into either of
Appl. Environ.
these categories.)
Microbiol. 55:14351446, 1989
Early Origins
Bacteria are among the earliest forms of life that appeared on Earth billions of years ago. Scientists
think that they helped shape and change the young planet's environment, eventually creating
atmospheric oxygen that enabled other, more complex life forms to develop. Many believe that
more complex cells developed as once free-living bacteria took up residence in other cells,
eventually becoming the organelles in modern complex cells. The mitochondria (mite-oh-con-dreeuh) that make energy for your body cells is one example of such an organelle.
What They Look Like
There are thousands of species of bacteria, but all of them are basically one of
three different shapes. Some are rod- or stick-shaped and called bacilli (buhsill-eye).
Others are shaped like little balls and called cocci (cox-eye).
Others still are helical or spiral in shape, like the Borrelia pictured at the top
of this page.
Ball-shaped
Streptococci
Simonson, ASM
MicrobeLibrary
Some bacterial cells exist as individuals while others cluster together to form
pairs, chains, squares or other groupings.
Where They're Found
Bacteria live on or in just about every material and environment on Earth
from soil to water to air, and from your house to arctic ice to volcanic
vents. Each square centimeter of your skin averages about 100,000
bacteria. A single teaspoon of topsoil contains more than a billion
(1,000,000,000) bacteria.
Bacteria that live in guts
of surgeon fish
Courtesy Norm Pace
Some bacteria move about their
environment by means of long, whip-like structures called
flagella. They rotate their flagella like tiny outboard motors to
propel themselves through liquid environments. They may also
reverse the direction in which their flagella rotate so that they
Bacterium with flagella
Harwood, ASM MicrobeLibrary tumble about in one place.
How They Move
Other bacteria secrete a slime layer and ooze over surfaces like slugs. Others are fairly stationary.
What They Eat
Some bacteria are photosynthetic (foe-toe-sin-theh-tick)—they can make their own food from
sunlight, just like plants. Also like plants, they give off oxygen. Other bacteria absorb food from
the material they live on or in. Some of these bacteria can live off unusual "foods" such as iron or
sulfur. The microbes that live in your gut absorb nutrients from the digested food you've eaten.
Many more interesting facts about bacteria can be found throughout the Microbe website, so keep
clicking and reading.
Viruses
When is a life form not a life form? When it's a virus.
Viruses are strange things that straddle the fence between living and non-living.
On the one hand, if they're floating around in the air or sitting on a doorknob,
they're inert. They're about as alive as a rock. But if they come into contact with
a suitable plant, animal or bacterial cell, they spring into action. They infect and
take over the cell like pirates hijacking a ship.
© Jean Yves
Sgro
What They Are
A virus is basically a tiny bundle of genetic material—either DNA or RNA—carried in a shell
called the viral coat, or capsid, which is made up of bits of protein called capsomeres. Some viruses
have an additional layer around this coat called an envelope. That's basically all there is to viruses.
What They Look Like
There are thousands of different viruses that come in a variety of shapes.
Many are polyhedral <polly-hee-drul>, or multi-sided. If you've ever looked
closely at a cut gem, like the diamond in an engagement ring, you've seen an
example of a polyhedral shape. (Unlike the diamond in a ring, however, a
virus does not taper to a point, but is shaped similarly all around.) Other
viruses are shaped like spiky ovals or bricks with rounded corners.
Rotavirus
Courtesy CDC
Some are like skinny sticks while others look like bits of looped string. Some
are more complex and shaped like little lunar landing pods.
Where They're Found
Viruses are found on or in just about every material and environment on Earth
Ebola virus
from soil to water to air. They're basically found anywhere there are cells to
Courtesy CDC
infect. Viruses have evolved to infect every form of life, from animal to plant
and from fungi to bacteria.
However, viruses tend to be somewhat picky about what type of cells they infect. Plant viruses are
not equipped to infect animal cells, for example, though a certain plant virus could infect a number
of related plants. Sometimes, a virus may infect one creature and do no harm, but cause havoc
when it gets into a different but closely enough related creature. For example, the Hantavirus is
carried by deer mice without much noticeable effect on the rodents. But if Hantavirus gets into a
person, it causes a dramatic and frequently deadly disease marked by excessive bleeding.
Single-Minded Mission
Viruses exist for one purpose only: to reproduce. To do that, they have to take
over the reproductive machinery of suitable host cells.
Upon landing on an appropriate host cell, a virus gets its genetic material
inside the cell either by tricking the host cell to pull it inside, like it would a
nutrient molecule, or by fusing its viral coat with the host cell wall or
membrane and releasing its genes inside. Some viruses inject their genes into
the host cell, leaving their empty viral coats sitting outside.
Type A flu virus
Courtesy CDC
If a virus is a DNA virus, its genetic material then inserts itself into the host cell's DNA. If the virus
is an RNA virus, it must first turn its RNA into DNA using the host cell's machinery before
inserting into the host DNA. The viral genes are then copied many, many times, using the
machinery the host cell would normally use to reproduce its own DNA. The virus uses the host
cell's enzymes to build new viral capsids and other viral proteins. The new viral genes and proteins
then come together and assemble into whole new viruses. The new viruses are either released from
the host cell without destroying the cell or eventually build up to a large enough number that they
burst the host cell like an overfilled water balloon.
Other Virus-Like Things
Viruses may be referred to often as the smallest infectious things. But there are some smaller
contenders. Some of the agents of plant disease lack even a viral coat and are merely small strings
of plain, or "naked," RNA. These particles are called viroids. They are believed to be a more
primitive version of ordinary viruses.
But maybe viroids aren't the smallest infectious agents all.
Prions
Do you recall hearing about Mad Cow Disease? This is an ailment that affects the animals' brains
and is also called bovine spongiform encephalopathy <boh-vine sponge-ee-form en-sef-uh-la-puthee> because it makes the brain appear holey, like a sponge. There is a human form of this disease
called Creutzfeldt-Jakob <kroits-feld ya-cob> disease. Some scientists now believe these brain
illnesses are among a few diseases caused by an infectious agents called prions <pree-ons>. Prions
are not even DNA or RNA, but simply proteins. They are thought to be misshapen or abnormal
versions of proteins normally found in animals or people. Very little is known about prions.
Scientists suggest that they spread when a prion comes into contact with the normal version of the
protein and causes the normal protein to change shape and become a prion, too. Visit All About
Prions in the News section to learn more details about prions.
Many more interesting facts about viruses can be found throughout this website. You can also find
a lot more detailed information and several photos of viruses at Hidden Killers: Deadly Viruses.
And you'll find a great collection of virus portraits at The Big Picture Book of Viruses.