In herpes litigation, the claims against partners have ranged from those who sinned by omission, keeping mum about their status, to those who, when asked if they had a sexually transmitted disease, lied. In a 1984 opinion in a herpes case, a California appellate court acknowledged that while rulings on bedroom behavior infringed the right to privacy, public-health-policy concerns loomed larger. Courts have decided that if someone is infected, aware of it and sexually active, that person has a duty to inform a partner, who by extension, has a right to know.

These rulings, said Lawrence Gostin, executive director of the American Society of Law and Medicine, suggest that judges believe that if someone doesn't tell, then "it's almost as if you're talking about a sexual assault, because without that information, a person can't give true informed consent to intercourse."

MR. GOSTIN, who teaches health law at the Harvard School of Public Health, closely follows AIDS litigation. So far, he said, there have been fewer than 20 suits nationwide claiming that a partner failed to disclose that he or she was H.I.V.-positive. But while the herpes claims were primarily made by partners who had been infected by former companions, the AIDS claims have so far addressed behavior that created a risk of infection, whether or not transmission occurred. The best-known of these cases is the one of Marc Christian, the companion of Rock Hudson, who won $5.5 million from the actor's estate even though he did not test positive for the virus.

In fact the bulk of AIDS court cases that address risky sexual behavior have not been civil suits, but criminal state prosecutions. Mr. Gostin estimated that more than 300 criminal cases have been pursued around the country, and that the number is increasing. In the early years of the AIDS prosecutions, defendants were tried under traditional felony statutes, like attempted murder and assault with a deadly weapon.

Such cases were hard to win, because prosecutors had trouble demonstrating intent, a key element in these crimes. There have been a few wild-card examples of H.I.V.-positive people who announced that they deliberately intended to harm victims by infecting them through sexual activity. But Mr. Gostin said, "It's hard to prove that if someone makes love with you that their intention is to kill you, because that's a highly indirect means of going about it."

And so, states began to craft criminal laws that specifically prohibited reckless sexual behavior by a person infected with the AIDS virus. In a 1990 law, the Federal Government even made the release of funds for the care of AIDS patients conditional in part on whether a state had adequate laws to prosecute H.I.V.-infected people who intentionally exposed others to the virus.