The Risk to Heterosexuals

By Dick Thompson

The history of medicine offers a harsh lesson about the deadly impact of sexually transmitted disease. The lethal form of syphilis was first introduced into the known world in Barcelona in 1494. Twelve years later, in an era when travel was difficult, the disease had reached China, killing millions along the way.

Five years into the AIDS epidemic, the deadly virus has already achieved a worldwide reach, and is threatening to break out in the general population. Two weeks ago, the U.S. Surgeon General warned that the threat of AIDS has made casual sex hazardous to anyone's health. The National Academy of Sciences echoed that admonition last week, predicting that heterosexual transmissions of the disease will increase sevenfold by 1991. Says Margaret A. Fischl, director of the AIDS program at the University of Miami: "Anyone who is sexually active, visits prostitutes or has casual sex needs to be concerned. The only safe sex is no sex."

Despite the alarm, many AIDS experts are quick to point out that not all heterosexuals are equally vulnerable. "Any sexually active individual in this society is at some risk, but the risk is different for different people," says Gerald Friedland, associate professor of medicine at New York City's Montefiore Medical Center and a specialist in the study of AIDS transmission. Nationwide, the Centers for Disease Control (CDC) has recorded 999 cases of heterosexually acquired AIDS. Less than half (446) of the victims acquired the disease in the U.S.; most of those are women who were associated with drug users.

Although researchers are racing to understand the disease, they still know very little for certain about how AIDS is transmitted, especially among heterosexuals. "We don't know all the mechanisms," says James Chin, chief of the infectious-disease branch of the California health services department. "We do know grossly, by observation, that it is bi-directional" -- in other words, that men and women are each able to spread the virus.

Although the AIDS virus has been found in saliva and tears, almost all AIDS transmission results from contact with the semen or blood of an AIDS victim. In semen, the virus rides as a passenger, probably in the disease-fighting white blood cells in the fluid. During intercourse, the white blood cells containing the AIDS virus alight on the mucous membranes inside the rectum or the vagina. Unlike the skin, which is an efficient barrier to the virus, the mucous membrane is a much thinner tissue and is more susceptible to infection. If microscopic tears occur in the membranes during sexual contact, these may act as passageways for the virus. But some studies in animals indicate that even tears may not be necessary for transmission.

Women transmit AIDS to men far less frequently. The CDC says only one- quarter of all U.S. heterosexually acquired AIDS cases are men. A woman's blood or secretions could infect a man during intercourse; the penis also has mucous membranes, but it is probably exposed to less virus. The risk from oral sex appears to be much lower than from either anal or vaginal intercourse.

Although questions still remain, researchers believe they have found some patterns in the heterosexual spread of the disease:

-- AIDS passes more easily to victims of other sexually transmitted diseases. "This has been a common denominator in all our cases," says Thomas C. Quinn, a senior investigator at the National Institutes of Health who has been studying the disease in Africa, where syphilis, gonorrhea, herpes and chancroid are common in AIDS victims. Quinn has observed a similar pattern in New York City and Miami. "Genital inflammation may increase susceptibility," says Quinn. "The virus may gain access via breaks in the skin due to sexually transmitted diseases."

-- It is rare (though not impossible) for a single exposure to the AIDS virus to produce an infection. In most cases, repeated exposure over time appears to be necessary. AIDS "may be more difficult to transmit than herpes," says Montefiore's Friedland. "A number of pieces of information suggest that you need a large amount of virus." Studies of the sex partners of AIDS victims -- people who have had hundreds of sexual encounters with carriers of the virus -- show that 40% to 50% do not become infected with the virus.

Nevertheless, although AIDS may be difficult to acquire, it is impossible to cure. At the very least, AIDS now threatens anyone who is promiscuous. The risk of the illness, said one AIDS researcher, "doesn't mean an end to sexual life. It means a rethinking of it."