Penny Arnold, age 13, of Marquette, Mich., for her question:
JUST WHAT EXACTLY IS A VIRUS?
A virus is a microscopic organism that lives in a cell of another living thing. Viruses are the smallest and simplest form of life and they are also a major cause of disease.
Some viruses infect humans with such disease as the common cold while other infect animals or plants. Viruses produce disease in an organism by damaging some of its cells.
Viruses are so primitive that many doctors consider them to be both living and nonliving things. By itself, a virus is a lifeless particle and cannot reproduce. But inside a living cell, a virus becomes an active organism that can multiply hundreds of times.
Viruses are shaped like rods or spheres. Most are so small that they can be be seen only with an electron microscope, which magnifies them by thousands of times.
A Dutch botanist named Martinus Beijerinck began the study of viruses in 1898 when he realized that something smaller than bacteria could cause disease. He named this particule "virus," a Latin word meaning "poison."
An American biochemist named Wendell Stanley showed in 1935 that viruses contain protein and can be crystallized. This research and many other studies eventually led to the development, during the 1950s, of vaccines for measles, poliomyelitis and other diseases.
Virologists, scientists who study viruses, demonstrated in the early 1900s that viruses can cause cancer in animals. During the 1960s and 1970s, and continuing into the 1980s, experiments with cancer viruses became an important part of biological research.
Viruses, unlike other organisms, are not made up of cells. Therefore, they lack some of the substances needed to live on their own. To obtain these substances, a virus must enter a cell of another living thing. It can then use the call's materials to live and reproduce.
Virus diseases in humans include chicken pox, colds, cold sores, measles, mumps, polio, rabies, influenza, hepatitis and smallpox.
The body protects itself from viruses and other harmful substances by several methods, all of which together are called the immunity system.
As an example, white blood cells called lymphocytes provide protection in two ways. Some lymphocytes produce substances called antibodies, which cover a virus protein coat and prevent the virus from attaching itself to the receptors of a cell. Other lymphocytes destroy cells that have been infected by viruses and thus kill the viruses before they can reproduce.
Lymphocytes do not start to produce antibodies until several days after a virus has entered the body. However, the body has additional methods of fighting virus infections.
For example, the body produces a high fever to combat such virus diseases as chicken pox and measles. The high fever limits the ability of the viruses to reproduce.
To fight colds, the body forms large amounts of mucus in the nose and throat. The mucus traps many cold viruses, which are expelled from the body by sneezing, coughing and blowing one's nose.
The body also makes a protein substance called interferon that provides some protection against all types of viruses.