Sometimes I think that viruses were created mainly to benefit the medical profession. "

You're not feeling well and you go to see your doctor. After an examination and a test or two, the doctor says, "You're sick all right. You have an infection. But it isn't a bacterial infection, it's a viral infection. So there's no point in giving you antibiotics. Just go home and take it easy until you feel better." Meaning, "We're not quite sure what's wrong with you, but we do know we can't give you anything to cure it."

Are viruses and bacteria really so different? On the face of it, they have a lot in common. They exist in large numbers everywhere, some forms serve as the agents for disease, and they are too small to be seen without a microscope. On closer inspection, however, viruses are much more mysterious objects than bacteria.

First, although both are tiny, viruses are orders of magnitude smaller. The largest known bacterium is relatively huge, a bloated object as big as the period at the end of this sentence. Bacteria are complete living organisms, which reproduce themselves given only a supply of nutrients.

By contrast, a virus is a tiny object, often less than a hundred-thousandth of an inch long. It is no more than a tiny piece of DNA or RNA, wrapped in a protein coat, and it cannot reproduce at all unless it can find and enter another organism with its own reproducing mechanism. It is different enough from all other life forms that some biologists argue that viruses are not really alive. Certainly, they do not fit into any of the known biological kingdoms.

The way in which a virus reproduces is highly ingenious. First, it must find and penetrate the wall of a normal healthy cell, often with the aid of a little tail of protein that serves as a kind of corkscrew or hypodermic syringe. Once inside, the virus takes over the cell's own reproducing equipment. It uses that equipment to make hundreds or thousands of copies of itself, until the chemical supplies within the cell are used up. Then the cell wall bursts open to release the viruses, which go on to repeat the process in another cell. Viruses are, and must be, parasitic on other life forms. They are the ultimate "Man who came to Dinner," who does not leave until he has eaten everything in the house, and also killed his host.

This explanation of what a virus is and does leads to a bigger mystery: Since a virus totally depends for its reproduction on the availability of other living organisms, how did viruses ever arise in the first place?

Today's biology has no complete answer to this question. However, it seems to me that the only plausible explanation is that viruses were once complete organisms, probably bacteria with their own reproducing mechanisms. They found it advantageous to invade other cells, perhaps to rob them of nutrients. As time went on, the virus found that it could get by with less and less of its own cellular factories, and could more and more use the facilities of its host. Little by little the virus dispensed with its cell wall and its nutrient-producing facilities, and finally retained only the barest necessities needed to copy itself. What we see today is the result of a long process of evolution, which could perhaps more appropriately be called devolution. The end result is one of nature's most perfect creations - reproduction reduced to its absolute minimum.

The virus is a lean, mean, copying machine. It may be a comfort to remember this the next time that you are laid low by what your doctor describes as a viral infection. And we can take greater comfort from the fact that, as our understanding increases, twenty years from now we should have "viral antibiotics" to tackle viruses and have us back on our feet within 24 hours.

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