health

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
Moreover, 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, whereas 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, which is
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.