Virology: Search for Essential Factors In Causes of Human Cancer

Researchers who are probing the mysteries of the relationship between viruses and human cancer may be tackling the most difficult job in all of medicine. It would be tough enough if their task involved whole viruses, most of which can not be seen and can be photographed only with the electron microscope. But cancer research must make even more minute explorations inside viruses; it must chart the behavior of molecules in a no man's land between the living and the nonliving.

A few elements in the mystery have been clarified, says ''Viruses and Cancer." a progress report published this week by Manhattan's Sloan-Kettering Institute for Cancer Research. In many of the cancers, including leukemias of domestic fowl and laboratory animals, a virus is an essential factor. But to say that a virus causes the cancer may be an oversimplification. The tubercle bacillus is the one essential factor in tuberculosis, but mil lions of people carry the bacillus without ever developing the disease. By analogy, researchers argue, it may be that viruses, or viruslike particles of whatever origin, are essential factors in human as well as in animal cancers. But it takes something else as well to bring on the disease, even though the virus particles may have been harbored for half a lifetime.

Not from Air Alone. Another seeming certainty is that no matter how viruses may be involved, cancer is not an infectious disease in the ordinary sense. Nobody catches lung cancer because a victim of the disease coughs in his face. From animals it appears that something like a virus, plus some sort of physical or chemical irritant, may be needed to bring on the disease. Mice do not get lung cancer from polluted air alone, nor from influenza virus. But they may develop something remarkably like human lung cancer if they are both infected with flu virus and exposed to air-polluting chemicals.

A dozen different viruses have been found to cause cancer in mice, and they show a bewildering variety of behavior. Some are clearly inherited. One is passed on from generation to generation in mouse-mothers' milk, so daughter mice develop breast cancer. A male mouse may be a healthy carrier of this virus and infect a female with which he is mated.

Mice that show no signs of harboring a leukemia virus may develop the fullblown disease, and produce the virus plentifully, after they are exposed to X rays.

Still more confusing are the crossovers between species. Millions of monkeys carry a virus which apparently does them no harm. But this virus, known as SV-40, may cause tumors if injected into hamsters. Viruses found in human tissues and in some rat cancers make hamsters bear deformed young with features resembling human mongolism. One virus that normally causes only grippe in man will cause cancer in hamsters.