Viruses are familiar from the common diseases they cause: colds and flu, for instance. But what are they, and how do they cause sickness?
A virus can be thousands of times smaller than a bacterium,
small enough to pass through most filters made to trap bacteria. Viruses
are found everywhere in nature, even in harsh environments like deserts and
polar seas, and thousands of feet underground. They also make up the bulk of
organic matter in the sea. They infect plants, animals, bacteria and humans.
Scientists estimate that there are millions of types of
viruses, most not yet discovered. So far, each type of virus that has been
discovered has its own unique genetic makeup. This means that viruses may
represent the largest reservoir of genetic material on earth. Viruses also may
create new combinations of genetic material as they reproduce. This means that
they create new or mutant versions of themselves.
Unlike living cells, viruses cannot on their own carry out
the biochemical processes needed to reproduce. They must be inside a living
cell to function and produce more viruses. But viruses are very specific about
what type of cell they need.
How a virus infects a cell
Some viruses can remain outside a cell for a long time.
Others can survive only in certain conditions. The virus's capsid protects the
virus when it is outside a cell. Some viruses have capsids that are resilient
and can withstand different environmental conditions. Others are fragile. The
capsid also determines the path by which the virus enters a living organism. It
also identifies the type of cell in the body that will host the virus.
Viruses usually infect only one type of cell. Once a virus
finds the appropriate cell, it attaches itself to the cell wall. The virus then
either enters the cell, or injects its genetic material (and enzymes, if it
carries them) into the cell. Once inside the cell, the viral DNA or RNA and
viral enzymes use the host cell’s own machinery to produce copies of the virus.
These newly created copies leave their host cell by exploding out of it --
killing the host cell -- or breaking through the cell wall in a process called
budding. The new viruses then find and infect other host cells.
Viruses can stay in the body area or organ they first
infect, or they can spread. Viruses that cause hepatitis,
for example, infect the liver and remain there. The measles virus and
varicella-zoster virus enter through the respiratory tract and spread to lymph
nodes, skin and other organs. Viral infections can damage body tissues in
several ways. They can interfere with the normal processes of the host cell,
kill the host cell by exploding out of it or trigger the immune system's
response.
In people with a healthy immune system, most common disease
viruses produce infections that last from seven to 14 days. Some viruses,
however, can cause chronic infections. Others lie undetected in the body and
cause symptoms at a later time, called a latent infection. In a chronic
infection, the virus reproduces and causes effects for an extended time,
perhaps for a person's entire life. Hepatitis B
and C viruses cause a chronic infection. In a latent viral infection, the
virus's DNA or RNA rests harmlessly in the host cells and does not reproduce. If
the virus is eventually activated, it begins to reproduce and damage body
tissues. Varicella viruses are examples of viruses that cause latent
infections. The varicella-zoster virus remains in the body after causing the
initial infection known as chicken pox.
After the initial infection, it enters the nerves and
travels to base of the spine, where it remains dormant, not reproducing and not
causing tissue damage. If it is re-activated, it travels through nerves to the
skin, where it causes the blister-like lesions of shingles.
The lesions appear along the route that the affected nerve follows underneath
the skin. The virus then returns to its dormant state.
Treating infections
Outside the body, viruses can be killed by detergents,
bleach, organic solvents such as ether or chloroform, and ultraviolet light.
Inside the body, the immune system provides defense by
producing antibodies against specific viruses. Antibodies are made when the
immune system first encounters a virus. The body builds an antibody specially
designed to prevent that particular virus from attaching to new cells. Once an
antibody is made for a specific virus, the immune system usually continues to
make it, but in much smaller quantities, even if there is no current viral
attack. If the immune system encounters that virus again, its response will be
faster because it does not have to build a new antibody. It simply makes more
of the ones it already has. This is called immunity.
You can develop immunity to fight a future viral infection
in two ways. You can catch the virus or get a vaccination. Vaccines are made
from a killed or inactivated form of the virus or from harmless parts of a
viral capsid grown in a laboratory. These substances contain just enough of the
virus to trigger the immune system to build an antibody, but not enough to
cause a serious infection. Vaccines exist for these viruses: chicken pox, shingles,
measles, mumps, rubella, hepatitis A, hepatitis B, yellow fever,
human papillomavirus, rabies, influenza, polio, Japanese encephalitis and
rotavirus.
Another of the body’s natural defenses against viral
infections is a family of proteins called interferons. Interferons also
fight bacterial infections and tumors. They do not kill viruses, but they
activate other immune responses in the body, including processes in host cells
that stop the virus's activity. Interferons can also be made commercially and
injected into the body to boost the immune system response.
Antiviral medications
Once a virus is inside a host cell, it is difficult to kill
or damage it without killing or damaging the cell. Because of this, scientists
have developed drugs that interfere with a virus's functions rather than
killing it outright. Antiviral drugs have been developed that prevent the virus
from attaching to a host cell, entering the cell, reproducing within a cell, or
releasing newly formed viruses. The drugs amantadine and rimantadine, for
example, work by preventing the virus from entering the cell; the drug
acyclovir blocks viral reproduction within the cell. Two newer drugs for the
treatment of influenza, zanamivir and oseltamivir, block the release of newly
formed viruses from the host cells, preventing their spread to other host
cells. Protease inhibitors, used in treatment of HIV,
work by blocking an enzyme the HIV virus
uses to make copies of itself.
Antibiotics,
which are prescribed for bacterial infections, don't work against viruses. This
is because antibiotics are
designed to interfere with biochemical reactions bacteria need to survive.
Viruses don't have these same biochemical reactions.
Several strains of HPV have been identified. The vaccine
protects against strains 16 and 18, which cause about 70 percent of cervical
cancers, and against types 6 and 11, which cause about 90 percent of genital warts.
The vaccine does not protect against HPV strains 31 or 45, which can also
cause cervical cancer. The vaccine, which does not contain live
virus, is approved for females ages 9 to 26.
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