COMMENTARY

THE PEOPLE reading this column will be the next target for AIDS education. Whether or not they will be the next target for AIDS remains to be determined.

The question often phrased, "Will the AIDS virus spread to the 'general population'?" is really a poorly stated mix of two other questions. First, what are the characteristics of a population that can sustain infection with the AIDS virus? And second, who--besides gay men and intravenous drug users--shares these characteristics?

A vulnerable population must have suitably efficient routes of transmission. Potentially infectious contact between members must occur often enough to offset the rate at which the disease--or the behavioral changes it entails--removes those who are infectious.

Too little is known to permit a conclusion as to whether these conditions are met by heterosexual people in the United States. Authorities, however, believe that heterosexual contact is instrumental in the spread of the AIDS virus in central Africa.

Whether the disease will be passed from one member of a population to another is the product of two risks: (1) that an infected person will come into contact with someone who is not infected, and (2) that whatever they choose to do together is capable of transmitting the infectious agent. In practical terms, "low risk" behavior may become "high risk" as the infection becomes more common.

CURRENTLY, THERE is good reason to believe that the AIDS virus is most efficiently transmitted in two ways. Through the sharing of needles, (but the practice apparently is much less hazardous when the sticks are minor and blood is not passed through the barrel of the needle). And second, through rectal or vaginal exposure to ejaculate carrying the virus.

A third situation, exposure of the penis to virus carried in vaginal or rectal fluid is probably less risky, but the risk is not zero. In any case, the cumulative risk of acquiring infection through repeating the first two events--and possibly also the third--is high.

Once infection is established, the risk of developing AIDS is about 15 to 25 percent within six years and at least that for developing virus-related symptoms outside the standard definition of AIDS. Although there is no data on infection that last longer than six years, there is no reason to believe that the risk of developing symptoms will diminish with time. People with symptoms of infection have been observed only to get worse or stay the same; they don't get better as time goes on.

When will we know that some groups of heterosexual people are at "high risk" of developing AIDS? Currently most heterosexually infected people attribute their infection to contact with intravenous drug users or bisexual men. If the time comes when most new heterosexual cases are contracted from other members of the "lower risk" population, we'll know that the epidemic has become established in that population and no longer needs to be imported.

Even if the world were simple and AIDS was restricted to intravenous drug users and heterosexuals and their behavior were uniform and predictable, there would be no way to estimate accurately the time at which most cases of AIDS would result from heterosexual-to-heterosexual contact. Depending on the assumptions, heterosexual-to-heterosexual contact could account for the majority of AIDS cases in six years, 10 years, or never.

The crucial questions will not be answered soon, and public fears will undoubtedly mount as the incidence of AIDS continues to rise in all categories of people at risk. The point is to bear in mind that some activities are safe even if repeated often with an infected partner. Others activities that have been "safe" are not now that the virus has entered new populations. Unfortunately, nobody knows for sure which is which.

Victor De Gruttola is an instructor in medicine at the Harvard Medical School's Channing Laboratory. William Bennett is the editor of the Harvard Medical School Health Letter.