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.
[Karl Note: The
truth is that a virus can have any shape that
any collection of garbage would reveal!
No science here!]
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.
[Karl Note:
"Most scientists" say that the virus is
alive? Well, then let me see what is the
definition for "life" used by these so-called
"scientists!" There is none, other
than, as I have written elsewhere, the theory
that man rose from the mud, a spontaneous
action of non-living chemicals. Once
you have given up on God (but never admit it)
you can then engage in all the psychiatric
damage done by those doctors -- who, also,
believe man is nothing more than a meat body
-- motivated by shock and stimulus!
Poor trash instead of science.!]
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.
[Karl Loren:
Explain to me, if you can, how a virus "can
enter" through the nose, mouth, etc.???
I would agree that YOU, a being who is alive,
can breathe in air containing viruses -- life
is cause. Stones are not!]
|
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.
|
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.
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