Dr. Frank Ryan, a physician and the author of a book about tuberculosis, has produced unsettling new theories about killer viruses which, says one informed professor, contains the essences of the story of AIDS.

Beyond AIDS, Dr. Ryan wonders, are there are possibly other viruses lurking, including some much easier to catch? The question has been asked before. Joshua Lederberg, a geneticist at Rockefeller University and a Nobel Laureate, asks about the possible existence of a virus as deadly as Ebola, but with a lengthy incubation period, such as has H.I.V., that could be circulated through droplets, possibly oozing from noses and mouths as happens with the onset of head colds.

Dr. Ryan answers that such a virus may indeed exist and indicates that humanity may well be on the way to catching it. Earth's varied species respond in different ways to the presence of certain unknown viruses. He calls such viruses "aggressive symbionts" and likens their habitats to those of ants who are sheltered in tropical bushes. Once aggravated, as passers-by unwittingly trample the plants where they reside, these ants come forth to defend themselves. In a similar manner, other species, whose presence remains unsuspected and who are hidden in tropical rain forests now under human assault, may send their own protective viruses to defend themselves.

Humans, in fact, carry a host of viruses that have few deleterious effects. These include the almost universal presence of Herpes simplex, which is likely to surface with mere cold sores in a few who are infected. But such contagion, while benign in homo Sapiens, could be deadly to the members of other species. A friendly kiss given by a person to a member of one the ape families, for example, might pass to that ape a more virulent infection. In like manner, viruses carried with no ill effects on them by rats, cows, and other animals can, with ease, kill human beings.

This, believes Dr. Ryan, is how H.I.V.-1 and H.I.V.-2 were possibly transferred respectively from a chimpanzee and a monkey. Humanity's assaults on the rain forests, he believes, now puts us all in extreme mortal danger, not simply because we are losing exotic sources for new medical usages, but because these same forests contain uncounted millions of species with which we are not yet familiar. They will no doubt defend themselves, he says, issuing protective viruses that will kill the human invaders--those trampling the bushes--and will spread world-wide, now that rapid transportation allows the tiny, deadly creatures their conduits.

The above scenarios are contained in Dr. Ryan's new book, "Virus X: Tracking the New Killer Plagues--Out of the Present and Into the Future" (Little Brown & Company). The doctor's frightening suggestions can not be classified, unfortunately, as mere science fiction. Too much of what he foresees has its roots in scientific research.