Bill Kraus drives slowly by the bathhouse parking lot and counts the cars. There are only eight of them tonight, eight cars in a lot that has room for at least 30.

For Kraus, a congressional aide and a leader in the gay community, the evidence of bad business for a bathhouse --where sex with strangers is the major commodity--is a somber, welcome sign. The gay men in this city are beginning to respond to the threat that appears on posters and pamphlets almost like warnings on cigarette packages: the threat of AIDS.

Acquired Immune Deficiency Syndrome is now a full-fledged epidemic in America, but particularly in the gay community. Of the 1,641 confirmed cases, 1,140 are identified as homosexual or bisexual. In San Francisco, where the homosexual community is the most concentrated and politically powerful in the country, and where the homosexual life style is often highly promiscuous, 90 percent of the 250 AIDS victims have been gay men.

The people who have AIDS stare at a bleak set of statistics. All those who got it three years ago are dead. Seventy-five percent of those who got it two years ago also are dead. In fact, earlier this day, Bobby Campbell, a lanky former nurse who came down with AIDS in 1981, when he was 29, described himself with bleak humor as "the AIDS poster boy," for the simple reason that he is still alive.

There is no way to overstate the effect of AIDS on the gay community. Nearly everyone knows someone who has it or died of it. Moreover, this minority group, persecuted for its sexuality, barely out of a closet full of demons, is now faced with a disease that is transmitted through semen or blood.

As Kraus put it, "When I first heard about AIDS, I thought, 'Oh, God, they've finally found a disease for the diseased.' It rekindles in the psyche all the hateful propaganda that you are sick."

Many in the gay community share those vivid memories and experiences of being labeled "sick" and "untouchable." In some terrible irony, they feel a medical stigma replacing the old social stigma.

The fear of being persecuted again, this time for carrying a "gay plague"-- although the disease is not believed to be caught through casual contact-- leads to denial and paranoia. Men such as Kraus who tried to convince gays of the need for precautions were labeled by some as "sexual fascists" or "homophobes." There are still stickers in the telephone booth near Castro Street, the heart of the gay district, insisting that AIDS "comes from government laboratory, not your life style."

But even for the majority of gay people who don't share "CIA conspiracy theories," the call to change their life styles sounded ominously like a medical form of the old repression. The entire history of open gay life is short and tenuous. It's still less than a decade since the Twin Peaks became the first gay bar in San Francisco to have glass windows.

Among many gay men, "liberation" was defined or confused as the right to have sex without restraint. Their sex life included a series of anonymous sexual encounters. Now, as one man put it, "we have to choose between life and life style."

Routinely, in one conversation after another, one man explains "the need to change my life style" and another says he has "dropped out of the fast lane" and a third describes "the return of courtship and romance." The choices range from celibacy to monogamy to condoms, but they are the subject of incessant, almost obsessive conversation here, particularly among "the worried well."

"There's never been anything that has had such a dramatic impact on people's behavior," says a local writer. "A year ago a fashionable gay man in the Castro area was promiscuous and used drugs. Today those are the people you want to stay away from." As Kraus describes the change in his community, "It's sort of like going down the freeway 65 miles per hour and throwing the car in reverse."

Says 31-year-old Bobby Campbell, who is living with AIDS: "Knowing someone's middle name does not protect you." But the reassessment of the culture seems to go beyond precaution.

Listening to gay men, I hear a mild nostalgia for the old excess, as if those had been fraternity days, and yet a strong sense that it is better to move on. AIDS, like some fatal herpes, has given homosexuals in the fast lane a reason to grab the emergency brake.

"If there is anything positive about this horrible situation, assuming the great majority of us survive, it will have been a crisis that makes people stronger," says Kraus. "I see it de-trivializing sex. I think that will be permanent. I think there's room in human society and gay society for people to be kinder and warmer to each other."

With luck and a cure, this may be the peculiar legacy of a dreadful disease.