Download Virus - KICS Learns

Viral Infections
Viral infections occur when viruses enter cells in the
body and begin reproducing, often causing illness.
Viruses are tiny germs that can reproduce only by
invading a living cell.
How Are Viruses Different from
Bacteria?
Viruses are far smaller than bacteria. They are so
small that they could not be seen until the electron
microscope was invented in the 1940s. Unlike most
bacteria, viruses are not complete cells that can
function on their own. They cannot convert
carbohydrates to energy, the way that bacteria and
other living cells do. Viruses depend on other organisms for energy. And viruses cannot reproduce unless they get
inside a living cell. Most viruses consist only of tiny particles of nucleic acid (the material that makes up genes)
surrounded by a coat of protein. Some have an outer envelope as well.
Thousands of viruses
There are thousands of viruses, and in humans they cause a wide range of diseases. For instance, rhinoviruses cause
colds, influenza viruses cause flu, adenoviruses cause various respiratory problems, and rotaviruses cause
gastroenteritis. Polioviruses can make their way to the spinal cord and cause paralysis, while coxsackieviruses
(sometimes written as Coxsackie viruses) and echoviruses sometimes infect the heart or the membranes surrounding
the spinal cord or lungs. Herpesviruses cause cold sores, chickenpox, and genital herpes, a sexually transmitted
disease. Other viruses cause a variety of conditions from measles and mumps to AIDS.
The body's defense system
Most viruses do not cause serious diseases and are killed by the body's immune system—its network of natural
defenses. In many cases, people never even know they have been infected. But unlike bacteria, which can be killed by
antibiotics, most viruses are not affected by existing medicines. Fortunately, scientists have been able to make
vaccines, which help the body develop natural defenses to prevent many viral infections.
How Do Viruses Infect the Body?
Viruses can enter the human body through any of its openings, but most often they use the nose and mouth. Once
inside, the virus attaches itself to the outside of the kind of cell it attacks, called a host cell. For example, a rhinovirus
attacks cells in the nose, while an enterovirus binds to cells in the stomach and intestines. Then the virus works its
way through the host cell's outer membrane.
After entering the cell, the virus begins making identical viruses from the host cell's protein. These new viruses may
make their way back out through the host cell's membrane, sometimes destroying the cell, and then attacking new
host cells. This process continues until the body develops enough antibodies* and other defenses to defeat the viral
invaders.
Are Viruses Alive?
It would seem to be a simple matter to tell if something is alive. But biologists disagree on whether viruses are a
form of life.
Viruses lack certain features that other forms of life have. They cannot convert carbohydrates, proteins, or fats into
energy, a process called metabolism. They cannot reproduce on their own, but must enter a living cell and use the
host cell's energy. On the other hand, like all life forms, viruses do have genes made of nucleic acid that contain the
information they need to reproduce.
Biologists have an elaborate way of classifying every form of life. Each is grouped into a kingdom (such as the
Animal Kingdom) and smaller sub-categories called the phylum, class, genus, and species.
Bacteria and fungi each have a kingdom of their own, but viruses are left out of this system. Many biologists think
that, unlike the forms of life grouped into kingdoms, viruses did not evolve (develop) as a group. Instead, viruses may
have developed individually from the kind of cells they now infect—animal cells, plant cells, or bacteria.
*
antibodies are proteins made by the body's immune system to target a specific kind of germ or other foreign
substance.
Not all viruses attack only one part of the body, causing what is called a localized infection. Some viruses spread
through the blood-stream or the nerves, attacking cells throughout the body. For instance, HIV, the human
immunodeficiency virus that causes AIDS, attacks certain cells of the immune system that are located throughout the
body.
How Long Do Viral Infections Last?
In most types of viral infection, the immune system clears the virus from the body within days to a few weeks. But
some viruses cause persistent or latent* infections, which can last for years. In these cases, a person may get infected
and seem to recover or may not be aware of being infected at all. Then years later, the illness will occur again, or
symptoms will start for the first time. Viruses that can cause latent infections include herpesviruses, Hepatitis B and
C viruses, and HIV.
How Do Viruses Cause Illness?
Viruses can cause illness by destroying or interfering with the functioning of large numbers of important cells.
Sometimes, as mentioned earlier, the cell is destroyed when the newly created viruses leave it. Sometimes the virus
keeps the cell from producing the energy it needs to live, or the virus upsets the cell's chemical balance in some other
way. Sometimes the virus seems to trigger a mysterious process called "programmed cell death" or apoptosis (ap-opTO-sis) that kills the cell.
Some persistent or latent viral infections seem to transform cells into a cancerous state that makes them grow out of
control. It has been estimated that 10 to 20 percent of cancers are caused by viral infections. The most common are
liver cancer caused by persistent infection with Hepatitis B or Hepatitis C virus, and cancer of the cervix (the bottom
of a woman's uterus or womb), linked to certain strains of the human papillomavirus.
*
latent infections are dormant or hidden illnesses that do not show the signs and symptoms of active diseases.
Sometimes a viral illness is caused not by the virus itself, but by the body's reaction to it. The immune system may
kill cells in order to get rid of the virus that is inside them. This can cause serious illness if the cells being
There are thousands of kinds of viruses. Most consist only of tiny particles of genetic material surrounded by a coat
of protein and sometimes an outer envelope. Specific viruses attach themselves to the outsides of specific host cells,
and then work their way inside through the host's outer membranes. Once inside their host cells, the viruses
reproduce. The new viruses can destroy their host cells and then move on to attack new host cells.
killed are very important to the body's functioning, like those in the lungs or central nervous system, or if the cells
cannot reproduce quickly enough to replace the ones being destroyed.
An individual adenovirus viewed under an electron microscope. Viruses are so small that they could not be seen until
the electron microscope was invented in the 1940s. This one was photographed at 800,000 times its original size.
© Hans Gelderblom/Visuals Unlimited.
How Are Viral Infections Diagnosed and Treated?
Symptoms
Symptoms vary widely, depending on the virus and the organs involved. Many viruses, like many bacteria, cause
fever, and either respiratory symptoms (coughing and sneezing) or intestinal symptoms (nausea, vomiting, diarrhea).
Viral illnesses often cause high fevers in young children, even when the illnesses are not dangerous.
Diagnosis
Some viral infections, such as influenza, the common cold, and chickenpox, are easily recognized by their symptoms
and no lab tests are needed. For many others, such as viral hepatitis, AIDS, and mononucleosis, a blood sample is
analyzed for the presence of specific antibodies to the virus. If present, these antibodies help confirm the diagnosis. In
some cases, a virus may be grown in the laboratory, using a technique called tissue culture, or identified by its nucleic
acid, using a technique called polymerase chain reaction (PCR). Tests like PCR or tissue culture are used when
antibody tests are not precise enough or when the actual amount of a virus in the body must be determined.
Treatment
Viruses cannot be treated with the antibiotics that kill bacteria. Fortunately, a few drugs, such as ribavirin and
acyclovir, can control the spread of viral invaders without destroying host cells. Intense research to find better
treatments for AIDS has led to development of many drugs that help fight the virus. Unfortunately, none of these
drugs has been able to treat viral infections as effectively as antibiotics treat bacterial infections.
How Are Viral Infections Prevented?
Hygiene and sanitation
The first step in preventing the spread of viral infections is simply to practice good hygiene. This means washing the
hands often, and eating only food that has been prepared properly. It also means building and maintaining facilities
for getting rid of sewage safely and for providing clean drinking water.
Vaccination
Another important preventive measure is immunizing people against viruses. This involves giving people vaccines
that stimulate the immune system to make antibodies, proteins that target a specific germ. Vaccines to prevent
Hepatitis B, polio, mumps, measles, rubella (German measles), and chickenpox are usually given to babies and young
children in the United States. Vaccines also can prevent influenza and Hepatitis A.
What Is a 24-Hour
Virus?
When people have a mild illness—perhaps fever and an upset stomach, perhaps nausea and diarrhea—they often say
they have a "24-hour virus" or a "stomach virus." Many viruses can cause these kinds of symptoms, but there are
many other possible causes as well, including bacterial infection or bacterial food poisoning. People usually recover
from these brief or mild illnesses before doctors can do the tests that determine the causes. So a "stomach virus" may
or may not be a virus at all.
Vaccines are useful only against certain kinds of viruses. For example, the polioviruses that cause poliomyelitis
(polio), a great crippler of children in the past, are few in number and relatively stable. So it was possible in the 1950s
to make a vaccine that protects children from getting polio (although the illness still occurs in the developing world
where fewer children are vaccinated). On the other hand, influenza viruses change in minor ways every few years and
in a major way about every ten years, so a flu vaccine is useful for only a year or two.
One reason a vaccine for the common cold has never been developed is that there are at least a hundred different
rhinoviruses that cause colds, and so far it has not been possible to make a vaccine that works against all of them. A
similar problem with HIV, which has many different and fast-changing strains (variations), is one of several reasons
why progress toward an AIDS vaccine has been slow.
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 tightly-woven 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 single-celled, 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 subclassification 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 single-stranded 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 "closing-off" systems which
generate a walled-off area within the plant at the source of the infection. Bacteria can be protected from bacterialspecific 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.