Christine Peterson, age 12, of Peoria, 111., for her question?

What are viruses?

In the world of living things, a virus is smaller than small. A bacterium is a microscopic living thing because we can study it only through a microscope. Few viruses are large enough to be studied even under the most powerful microscope lenses. We need the electron microscope for most of them. To us, a flea is a tiny midget. To a flea, a bacterium is a midget and to the bacterium, a virus is a midget.

Bacteria are big enough to be filtered through porcelain, but viruses are small enough to escape through the fine pores of the porcelain: Each virus seems to be a large and complicated molecule made from the heavy protein chemicals found in the nucleus of a living cell. At one time, many experts believed that viruses were simply non living chemicals. Now we know that they are living things able to rest, thrive and multiply.

When these facts were known, we had to give up the idea that a virus is merely a complicated chemical. Some people then though of it as the simplest form of life   perhaps the very first step from which all the more complex forms of life have developed. This idea too had to be discarded. For the virus i s a parasite.

It thrives and multiplies only in the cells of more advanced plants and animals. Hence it could not have existed before the higher plants and animals developed. It is not, then, the first step in the history of life on earth. Some people believe that the virus has descended on the scale of life. At one time it may have been more highly developed, perhaps like a one celled plant or animal. It lost its ability to live a free and independent life when it became a parasite.

There is a wide variety of these little pests and none of them seem to be our friends, It is known that they are responsible for some 200 diseases of the plant and animal world and we can thank them for some 50 diseases that plague mankind. They cause our cold and chicken pox, smallpox and sleeping sickness, polio and mumps.

A virus enters living tissue through the cell walls. There it thrives and begins to multiply. In no time at all, a new generation of viruses leaves the cell and sets forth to attack other cells. These virus invaders cannot be destroyed by pQnicillin and the other wonder drugs which control germ bacteria.

However, the body has its own defenses against them. If you get a mild attack of a certain virus, the body may be able to build up an immunity against a 'future attack. This is why sensible people get vaccinated against smallpox and polio, The vaccine makes the body build up immunity from these viruses. Since rabies is also caused by a virus, it is only kind to have our pets infected against this dread disease.