Sometime soon, Dr. Bobby Caldwell, "St. Elsewhere's" plastic surgeon and resident hunk, is going to carry his increasingly undiscriminating promiscuity one step too far. He is going to get AIDS from one of his lady friends. She's not a prostitute, nor does she appear to be an intravenous drug abuser, according to the script of an upcoming installment of NBC's award-winning television show. Caldwell has never shown any hint of homosexuality. Promiscuity, the series seems to be saying, equals AIDS.

Is it true?

Nobody in the real world's mainstream medical community can say for sure, but the chances appear small. Yet, in any case, "St. Elsewhere" -- a kind of big-city "M*A*S*H" -- has landed itself plunk into a real-life ongoing medical controversy.

Indeed, as soon as NBC announced last month that actor Mark Harmon's character was going to depart the show as a victim of acquired immune deficiency syndrome, the network began getting calls and letters from AIDS experts pointing out that female-to-male transmission is considered highly rare.

Scriptwriters and producers consulted some AIDS research centers, including the National Institutes of Health, and decided that "there is enough of a body of evidence to support the plot line," according to a network spokesman.

The controversy over female-to-male transmission of AIDS is relatively new.

The so-far universally fatal illness, now known to be caused by a virus identified in 1983 by French researchers and in 1984 by Americans, was first seen in the United States and Europe as an epidemic almost exclusively limited to homosexual males, especially those with multiple partners and especially those who were the receptive partner in anal intercourse.

It soon became clear that at-risk populations also included intravenous drug abusers of both sexes (who transmitted the virus through the shared use of contaminated needles), recipients of blood transfusions and hemophiliacs who used certain blood products. Among heterosexuals, the virus was transmitted by infected men to their female partners and by infected pregnant women to their unborn babies.

A massive effort resulted in a screening test for blood donors, which has virtually eliminated the possibility of new infection from blood transfusions or blood products. The blood test detects an antibody that the body produces in response to the AIDS virus, which reveals exposure to the virus but does not by itself show whether a person has or will get AIDS.

And a wave of terror, akin to the panics associated with the black plagues of half a millenium ago, began to ebb as casual contact, even affectionate kissing, was ruled out by researchers as a means of AIDS transmission.

Researchers have found more evidence of heterosexual transmission of AIDS, especially in Africa, especially linked to promiscuity and prostitution.

In this country, heterosexually transmitted AIDS -- including both female-to-male and male-to-female transmission, or, as the experts put it, bidirectional -- accounts for only about 1 percent of AIDS cases, which now number about 16,000. The percentage of heterosexual cases has remained the same -- 1 percent of the total number of cases -- although the actual number has increased along with the number of people with AIDS.

Female-to-male transmission has been thought to be extremely rare, but in a recent issue of the Journal of the American Medical Association, a Walter Reed Hospital team headed by Dr. Robert R. Redfield reported on a small group of male AIDS patients who apparently acquired the disease through vaginal intercourse. Of the servicemen in this category, most described having more than 50 sexual partners in the past five years and several of them had more than 100, including prostitutes.

However, the report was criticized in a letter to the journal from Dr. B. Frank Polk, professor of epidemiology at the Johns Hopkins School of Public Health, and principal investigator of a federally funded four-city AIDS research program called SHARE -- Study to Help AIDS Research Efforts.

Polk noted that because disclosure of homosexuality or drug abuse was cause for automatic discharge from the military services, servicemen may be reluctant to reveal those activities and may instead claim heterosexual contact as the means of disease transmission. "Reporting bias," he wrote, "may account for the reported association between heterosexual promiscuity and AIDS."

He also said data suggesting bidirectional transmission of AIDS in Africa will not hold up. The apparent link of multiple partners, including prostitutes, to the transmission of AIDS may be "confounded by the medical reuse of inadequately sterilizied needles and syringes for administering therapy for the sexually transmitted diseases that are causally associated with heterosexual promiscuity."

Polk was among the scientists who wrote to Bruce Paltrow, executive producer of "St. Elsewhere," urging that the script be revised.

"The plot as it stands is very misleading," Polk wrote, "in that it suggests that this kind of transmission is common. In fact, if it happens at all, it is very uncommon.

"It is absolutely the wrong message. To present it would arouse undue concern and fear among a population already frightened by this disease."

A Hopkins spokesman added that "people really believe those characters. They're always saying, 'But I saw it on television, and they wouldn't say it if it weren't true.' It will just start again with a round of scare stories."

Another spokesman at the National Institute of Allergy and Infectious Diseases (NIAID), where much AIDS research is centered, noted that many of the calls received there are "guilt-motivated -- guys who saw a prostitute five years ago and are worried about giving it to their wives, really worried even though they have no symptoms at all."

Dr. Anthony Fauci, chief of NIAID, said the African studies and prostitute data are "strongly suggestive" of an increase in female-to-male transmission.

But at this point, because the prevalence of the virus antibody is so low in the U.S. heterosexual population -- 0.08 percent -- "the chances of an otherwise healthy, normal heterosexual person coming into contact with exposure in the normal, everyday socio-sexual interactions in this country are minuscule," he said.

On the other hand, however, "when one has contact with individuals in a risk group, or those having relations with people in the risk group, then you proportionally increase your chances of becoming exposed," he said. "It is a straightforward, uncomplicated numbers game."

Researchers do not yet know exactly how female-to-male sexual transmission may occur. Anal intercourse is considered an especially "efficient" mode of transmission, Fauci said, although "it now is clear that you don't need anal intercourse. It is not the only way."

Fauci distinguishes between "rational fear and irrational fear" of AIDS. "You should have a rational concern that you are increasing the probability of being exposed to the virus if you have sex with a prostitute," he said, "but it would be irrational to think that you should just refrain from sex.

"The middle ground would be that one can have a healthy sexual relationship with people heterosexually, but should be avoiding prostitutes, especially street prostitutes in big cities, i.v. drug abusers and partners having affairs with these people. And also having some discretion about not being promiscuous to the point of having a large number of partners, some of whom you have no idea what their sexual history was before they met you."

But the bottom line came from a National Institutes of Health spokesman who had taken some questions from "St. Elsewhere" scriptwriters.

"What do you know about Dr. Caldwell, anyway? Mightn't there have been a man among all those partners? After all, who knows what anybody is doing -- even on 'St. Elsewhere.' "