Wanted: A Host

Viruses are the cause of some of mankind's most deadly epidemic diseases--e.g., influenza, yellow fever, smallpox, infantile paralysis. But until a few years ago, nobody had ever seen a virus. Now, thanks to the electron microscope which makes them visible, biologists are able to study the viruses' submicroscopic world.

Viruses are the smallest known disease-producing organisms: the biggest of them is less than one-hundred-thousandth of an inch in diameter. Biologists once wondered whether a virus was a living organism or just an overgrown, active protein molecule. The dispute is still not entirely settled, but the electron microscope shows that many of them look and act like living things. At a recent American Medical Association symposium, leading U.S. virologists described an amazing variety of viruses, ranging from types that attack only bacteria to those that infect man.

All viruses are alike in one respect: they are parasites that can operate only in a living cell. But they differ greatly in size, looks and behavior. They also show astonishing individuality. Some are round, some shaped like rods, some have tails like tadpoles. A few, almost as complicated as bacteria, which are a higher form of life, even have partial enzyme systems to help digest their food. Most viruses are rabid specialists and choosy about what they invade. Some thrive only in plants, some only in certain animals, some only in man, some only in certain tissues; e.g., the influenza virus in man can exist only in the lining of the breathing apparatus (nose, throat, lungs, etc.) or the eye sac.

Though viruses have no known sex. they reproduce themselves, apparently under the same laws of heredity as living creatures. Through mutations, they constantly produce new breeds. But some strains remain remarkably pure. The mumps virus, for example, has not changed in thousands of years; mumps symptoms are still about the same as they were when first described by Hippocrates (circa 400 B.C.).

Live & Let Live. Because viruses move in a twilight zone between life and inanimate matter, scientists used to think that they might represent the primitive beginnings of life. Many experts now believe that it is the other way around. One of the world's top virus authorities, Australia's Dr. Frank M. Burnet, a champion of the evolution-in-reverse theory, contends that viruses may once have been bacteria and that they are steadily degenerating into more simple forms.

Dr. Burnet thinks that man has less to fear from viruses than from bacteria. An outstanding fact about viruses, says he, is that their well-being depends on the health of their host. Unlike bacteria and insects, which are often out-&-out rivals of man, viruses can live only as long as the human being they infect. Unfortunately for the host, viruses often commit suicide by killing the patient. But in the long run, says Burnet, the virus varieties with the best chance of survival are those that "live & let live." Many of the viruses that infect man have evolved into forms that produce low-grade infections, often so harmless that the host may not notice them. And a light infection may make human beings immune to attack by a more dangerous variety of the same virus.

The Vulnerable Minority. Many virus diseases that chronically attack man--measles, mumps, chickenpox, herpes (blistered lips)--are mild. Serious epidemics, Burnet says, usually arise 1) in a susceptible population that has not been exposed to the virus, or 2) from a virulent new virus produced by a change in an old form.

Isolated groups of men in Spitsbergen and Greenland, Burnet points out, generally withstand the arctic winter without illness but summer's first ship brings a violent epidemic of colds. Doctors think that vulnerable victims catch it from carriers who are immune through constant exposure. Even great flu epidemics like the 1918 pandemic, says Burnet, attack only a vulnerable minority of the population. And most flu epidemics quickly run their course, leaving the population immune, at least temporarily, to another epidemic.

Viruses, says Burnet, are an unhappy byproduct of civilization; they cannot survive in small or widely scattered populations. Because viruses multiply fast and change in unexpected ways, "new virus diseases of man may well arise in the future." But the chances against any new virus getting a strong foothold are a million-to-one.

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