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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 edition of HowStuffWorks, 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:
- Nucleic acid - Set of genetic instructions, either DNA or RNA, either
single-stranded or double-stranded (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.
 In the lytic cycle, the virus reproduces itself using the host cell's
chemical machinery. The red spiral lines in the drawing indicate the virus's
genetic material. The orange portion is the outer shell that protects
it.
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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):
- A virus particle attaches to a host cell.
- The particle releases its genetic instructions into the host cell.
- The injected genetic material recruits the host cell's enzymes.
- The enzymes make parts for more new virus particles.
- The new particles assemble the parts into new viruses.
- 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.
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:
- 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:
- 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.
- Viruses in the fluid that drips down your throat attack the cells lining
your throat and give you a sore throat.
- 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.
 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. |
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 above.
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:
- 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:
- 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. |