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The following packet contains a variety of informational resources
about viruses. You will be able to use these resources as we discuss
items in class, when you work on your assignments, and when you are
completing your assessment.
Information in this Packet has been compiled from the following
sources:
1. GLENCOE Science: An Introduction to the Life, Earth, and Physical
Sciences. Glencoe McGraw-Hill, Peoria, Illinois: Glencoe McGraw-Hill, 1999.
2. http://science.howstuffworks.com/virus-human.htm
3. http://www.accessexcellence.org/RC/VL/GG/examples_of_viral.html
4. http://people.ku.edu/~jbrown/virus.html
What is a Virus?
A virus (VI rus) is a particle that has things in common with both
living and nonliving things. Viruses are like living things in that they
are able to reproduce. But they can reproduce only inside a living
cell. Viruses are nonliving things because they don't grow, eat, or
respond to their environment. Viruses need living things to survive.
Once a virus attaches to a cell, it invades the cell and begins to
make copies of itself. Eventually, the cell occupied by the virus
explodes, releasing many new viruses. These new viruses then
enter other cells.
Examples of Viruses
What do measles, AIDS, colds, and chicken pox all have in
common? Viruses cause all of these diseases. While viruses
cannot be classified into any of the six kingdoms, they do
infect all kinds of living things. Viruses, such as the one
pictured in Figure 1, infect bacteria as well as plants and
animals. Some plant viruses destroy food crops such as
potatoes and tomatoes. Other types can cause cancers in
house cats and humans.
FIGURE 1This computer-enhanced
image is of a virus that
causes
influenza
in
humans. Influenza is
often accompanied by
fever and body aches.
Influenza is the scientific
name for the flu.
Viruses also cause cold sores and chicken pox in humans.
Humans can be infected with either of these viruses by
touching someone who already has one of these viruses.
Hepatitis B and HIV are viruses that can be passed from
person to person by blood and other body fluids and on
needles carrying the virus. Hepatitis B damages the liver. HIV
can lead to AIDS. AIDS is a disease that eventually destroys
the body's ability to fight off diseases that can be life
threatening.
Treating Viruses
Most viruses, like the one that causes the common cold, can’t be treated. Other viruses that
affect humans, however, can be prevented with vaccines. Vaccines cause the body to make
substances that resist particular viruses. Many states require vaccines for polio, measles, and
mumps. These diseases can affect children and adults. Unfortunately, the exact origin of
viruses is not known. Most scientists, however, think viruses were originally mutated genes
that somehow were able to escape the cells in which they formed. Viruses are so small that
they cannot be viewed with a light microscope. Scientists have used electron microscopes to
view these microbes.
How Viruses Work
by Craig C. Freudenrich, Ph.D.
Most of us at one time or another have had colds or the flu, and we are especially vulnerable during the cold
and flu season. The symptoms -- fever, congestion, coughing, sore throat -- spread through offices, schools
and homes, no matter where in the world we live. Colds and flu (influenza) are caused by viruses. Viruses
are responsible for many other serious, often deadly, diseases including acquired immunodeficiency
syndrome (AIDS), Ebola hemorrhagic fever, infectious hepatitis and herpes. How can viruses cause so
much trouble? What makes us so vulnerable to them, and what makes them spread?
In this article, we will explore the world of viruses. We'll talk about what a virus is, what viruses look like, how
they infect us and how we can reduce the risk of infection. And you'll learn why you feel so miserable when a
cold virus attacks your body!
What is a Virus?
If you have read How Cells Work, you know how both bacteria cells and the cells in your body work. A cell is a
stand-alone living entity able to eat, grow and reproduce. Viruses are nothing like that. If you could look at a
virus, you would see that a virus is a tiny particle. Virus particles are about one-millionth of an inch (17 to 300
nanometers) long. Viruses are about a thousand times smaller than bacteria, and bacteria are much smaller
than most human cells. Viruses are so small that most cannot be seen with a light microscope, but must be
observed with an electron microscope.
A virus particle, or virion, consists of the following:
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Nucleic acid - Set of genetic instructions, either DNA or RNA, either single-stranded or doublestranded (see How Cells Work for details on DNA and RNA)
Coat of protein - Surrounds the DNA or RNA to protect it
Lipid membrane - Surrounds the protein coat (found only in some viruses, including influenza; these
types of viruses are called enveloped viruses as opposed to naked viruses)
Viruses vary widely in their shape and complexity. Some look like round popcorn balls, while others have a
complicated shape that looks like a spider or the Apollo lunar lander.
Unlike human cells or bacteria, viruses do not contain the chemical machinery (enzymes) needed to carry out
the chemical reactions for life. Instead, viruses carry only one or two enzymes that decode their genetic
instructions. So, a virus must have a host cell (bacteria, plant or animal) in which to live and make more
viruses. Outside of a host cell, viruses cannot function. For this reason, viruses tread the fine line that
separates living things from nonliving things. Most scientists agree that viruses are alive because of what
happens when they infect a host cell.
How a Virus Infects You
Viruses lie around our environment all of the time just waiting for a host cell to come along. They can enter us
through the nose, mouth or breaks in the skin (see How the Immune System Works for details). Once inside,
they find a host cell to infect. For example, cold and flu viruses will attack cells that line the respiratory or
digestive tracts. The human immunodeficiency virus (HIV), which causes AIDS, attacks the T-cells of the
immune system.
Regardless of the type of host cell, all viruses follow the same basic steps in what is known as the lytic cycle
(see figure above):
1. A virus particle attaches to a host cell.
2. The particle releases its genetic instructions into the
host cell.
3. The injected genetic material recruits the host cell's
enzymes.
4. The enzymes make parts for more new virus particles.
5. The new particles assemble the parts into new viruses.
6. The new particles break free from the host cell.
All viruses have some type of protein on the outside coat
or envelope that "feels" or "recognizes" the proper host
cell(s). This protein attaches the virus to the membrane
of the host cell. Some enveloped viruses can dissolve
right through the cell membrane of the host because
both the virus envelope and the cell membrane are
made of lipids.
Those viruses that do not enter the cell must inject their
contents (genetic instructions, enzymes) into the host
cell. Those viruses that dissolve into a cell simply
release their contents once inside the host. In either
case, the results are the same.
On the Inside
Once inside the cell, the viral enzymes take over those enzymes of the host cell and begin making making
copies of the viral genetic instructions and new viral proteins using the virus's genetic instructions and the
cell's enzyme machinery (see How Cells Work for details on the machinery). The new copies of the viral
genetic instructions are packaged inside the new protein coats to make new viruses.
Once the new viruses are made, they leave the host cell in one of two ways:
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They break the host cell open (lysis) and destroy the host cell.
They pinch out from the cell membrane and break away (budding) with a piece of the cell membrane
surrounding them. This is how enveloped viruses leave the cell. In this way, the host cell is not
destroyed.
Once free from the host cell, the new viruses can attack other cells. Because one virus can reproduce
thousands of new viruses, viral infections can spread quickly throughout the body.
The sequence of events that occurs when you come down with the flu or a cold is a good demonstration of
how a virus works:
1.
2.
3.
4.
An infected person sneezes near you.
You inhale the virus particle, and it attaches to cells lining the sinuses in your nose.
The virus attacks the cells lining the sinuses and rapidly reproduces new viruses.
The host cells break, and new viruses spread into your bloodstream and also into your lungs.
Because you have lost cells lining your sinuses, fluid can flow into your nasal passages and give you
a runny nose.
5. Viruses in the fluid that drips down your throat attack the cells lining your throat and give you a sore
throat.
6. Viruses in your bloodstream can attack muscle cells and cause you to have muscle aches.
Your immune system responds to the infection, and in the process of fighting, it produces chemicals called
pyrogens that cause your body temperature to increase. This fever actually helps you to fight the infection by
slowing down the rate of viral reproduction, because most of your body's chemical reactions have an optimal
temperature of 98.6 degrees Fahrenheit (37 degrees Celsius). If your temperature rises slightly above this,
the reactions slow down. This immune response continues until the viruses are eliminated from your body.
However, if you sneeze, you can spread thousands of new viruses into the environment to await another host.
Lysongenic Cycle
Once inside the host cell, some viruses, such as herpes and HIV, do not reproduce right away. Instead, they
mix their genetic instructions into the host cell's genetic instructions. When the host cell reproduces, the viral
genetic instructions get copied into the host cell's offspring. The host cells may undergo many rounds of
reproduction, and then some environmental or predetermined genetic signal will stir the "sleeping" viral
instructions. The viral genetic instructions will then take over the host's machinery and make new viruses as
described above. This cycle, called the lysogenic cycle, is shown in the figure below.
In the lysogenic cycle, the virus reproduces by first injecting
its genetic material, indicated by the red line, into the host
cell's genetic instructions.
Because a virus is merely a set of genetic instructions surrounded by a protein coat, and because it does not
carry out any biochemical reactions of its own, viruses can live for years or longer outside a host cell. Some
viruses can "sleep" inside the genetic instructions of the host cells for years before reproducing. For example,
a person infected with HIV can live without showing symptoms of AIDS for years, but they can still spread the
virus to others.
Reducing the Spread
As discussed above, viruses can exist for a long time outside the body. The way that viruses spread is
specific to the type of virus. They can be spread through the following means:
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Carrier organisms - mosquitoes, fleas
The air
Direct transfer of body fluids from one person to another - saliva, sweat, nasal mucus, blood,
semen, vaginal secretions
Surfaces on which body fluids have dried
To reduce the risk of spreading or contacting viruses, here are things you can do:
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Cover your mouth or nose when you sneeze or cough.
Wash your hands frequently, especially after going to the bathroom or preparing food.
Avoid contact with the bodily fluids of others.
These practices are not foolproof, but they can help you reduce the risk of viral infection.
Medicines That Can Help
Contrary to popular belief, antibiotics have no effect on a virus. Most antibiotics interfere with the reproduction
of bacteria, hindering their creation of new genetic instructions or new cell walls. Because viruses do not carry
out their own biochemical reactions, antibiotics do not affect them.
Immunizations work by pre-infecting the body so it knows how to produce the right antibodies as soon as the
virus starts reproducing. Also, because viruses reproduce so quickly and so often, they can often change
slightly. Sometimes, mistakes creep into their genetic instructions. These changes might alter the protein coat
slightly, so one year's batch of vaccine might not be as effective against the same type of virus next year. This
is why new vaccines must be produced constantly to fight viral infections and prevent outbreaks.
You may have heard of outbreaks of Ebola virus or West Nile virus that have left many people dead. Influenza
has killed many people in the past (early in the 20th century), and debate rages over when the next major flu
epidemic will occur in the United States. Not all viruses are deadly. For example, people get colds all of the
time and do not die. However, even these seemingly harmless viruses can be deadly to a person who already
has a weakened immune system -- people with AIDS, cancer patients taking chemotherapy, elderly people or
newborns. We have to take care not to spread viruses to these especially susceptible people.
Lots More Information
Go to http://science.howstuffworks.com/virus-human.htm to access these links directly from the end
of the article.
Related HowStuffWorks Articles
How AIDS Works
How Cells Work
How SARS Works
How Blood Works
How Your Immune System Works
How Mosquitoes Work
More Great Links
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Institute for Molecular Virology
The Big Picture Book of Viruses
Plant Viruses Online
Introduction to the Viruses
All the Virology on the WWW
U.S. Center for Disease Control: Ebola Fact Sheet
U.S. Center for Disease Control: Special Pathogens Branch Home Page
Scientific American: Shaking the Ebola Tree
Outbreak: The Deadly Truth
USA Today Health: Ebola vaccine shows promise in new study
Ebola virus links on the Web
Access Excellence: Dr. Donald P. Francis Talks to Teachers About the Ebola Outbreak
FluNet
The Garry Lab Home Page - learn more about viruses in general
The Ebola Virus - well-done high school project
Ebola Information - National Center for Infectious Diseases
Ebola Outbreaks - Updates
New York City Department of Health - West Nile Virus Information
CDC: West Nile Virus
West Nile Virus Scare: What's behind the scare that closed Central Park?
AIDS Education Global Information System (AEGIS)
The Body: An AIDS and HIV Information Resource
Testing Yourself for HIV-1
Herpes.com
Mumps
Hepatitis
Measles
Rubella (German Measles)
Rabies - What You Need To Know
Virus Ultrastructure
Books and videos available from Amazon.com
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And The Band Played On: Politics, People and the AIDS Epidemic by Randy Shilts - a good discussion of the science
of epidemiology
And The Band Played On - video version with Matthew Modine and Alan Alda (among others)
A Dancing Matrix: How Science Confronts Emerging Viruses by Robin Marantz Henig
Virus Ground Zero: Stalking the Killer Viruses with the Center for Disease Control by Edwin Regis
Ebola: a Documentary of Its First Explosion by William T. Close
Virus Hunter: Thirty Years of Battling Hot Viruses Around The World by C.J. Peters, Mark Olshaker (Contributor)
Virus X: Tracking the New Killer Plagues by Frank Ryan
Virus Within: The Coming Epidemic by Nicholas Regush
Emerging Viruses: AIDS and Ebola: Nature, Accident or Intentional? by Leonard G. Horowitz, W. John Martin
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 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 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 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.