Jim Matthews, age 14, of Indianapolis, Indiana, for his question:

Are viruses plants or animals?

The nature lovers of ancient Greece began classifying living things at least 2,000 years ago. They took it for granted that nature's children are either plants or animals. So did later scientists, who classified life on earth in two large Animal and Vegetable Kingdoms. We everyday nature lovers also tend to think of living things as either plants or animals. However, modern biologists classify certain microscopic life forms in a sort of in between world    as neither plants nor animals.

Until the microscope was invented, observers had to depend solely on their eyes for surveying the world of nature. The first microscopes extended the limit of human vision    and tiny bacteria were discovered. These microscopic organisms, or some of them, have both plant and animal features    though they are generally classified in the plant kingdom. In recent years, improved microscopes revealed the shapes of the much smaller viruses. Later, the superior electron microscope revealed more of them    and also gave microbiologists a chance to study their structures and behavior patterns.

None of their features are typically plant or animal, which is why at present most biologists classify the viruses in an in between world of their own. The typical virus is less than a single cell, being merely a nucleus encased in a hard protein shell. The nucleus is stuffed with the biochemical, DNA    nature's blueprint that carries the behavior pattern of a species from generation to generation. As in the more complex organisms, each virus species bears a unique DNA pattern.

Hundreds of different viruses are known to modern science    and all of these micro organisms have one unfortunate feature. Every virus is 2 parasite. It must live and multiply in the living cells of a host plant or animal. Kept in a test tube and perhaps in other conditions, it may survive an indefinite period of inactivity. It becomes active only when  it enters the living cells of its particular host. Then the protein shell is discarded and the nucleic acid, DNA, begins issuing its orders. And virus DNA gives it orders to the DNA in the host cell. Instead of performing its normal duties, the living plant or animal cell begins producing duplicates of the invading virus.

In the 1890s, scientists got on the track of the viruses by suspecting that certain diseases were caused by microbes smaller than bacteria. The first such parasite to be identified was the tobacco virus, followed by other species that cause other diseases in plants and animals, including man. The simple organisms are quite unlike the many larger plant or animal parasites. Nor can we classify them in either catagory on the basis of whether they attack plants or animals. After all, the entire animal kingdom depends directly or indirectly on plant life. The study of viruses marches on and everything about them suggests that they constitute a third kingdom in the diverse pattern of life on earth.

Medical biologists have identified the viruses that cause polio and measles, dozens of cold viruses, and about SO others that cause diseases in man. Science has a number of ways to cope with them. One method is a vaccine that destroys or immobilizes a specific virus without damaging the living cells of the host. We have preventative vaccines against polio, measles and many other virus diseases. More will be added and someday we hope for anti virus vaccines to wipe out the common cold.