Under a new ruling by the California Supreme Court, a person who has reason to believe he or she has HIV may be sued by sexual partners should they become infected.
To knowingly pass on HIV is already illegal in California and people who do so may be sued for damages in state court, but the new ruling extends the state's view of when liability arises from the disease.
The chief issue before the court was whether an HIV-positive person who hadn't yet been tested for the virus could be held responsible, in a civil suit, for infecting a partner, and the answer is yes, at least in some circumstances.
The court has emphasised that the ruling applies to married or monogamous couples and not to those in casual relationships.
The decision considerably widens the scope for law suits against sexual partners over negligent transmission and represents the justices' view that people who have reason to know they are infected with HIV, should be subject to a lesser duty of care than persons who have reason to know they are infected with other sexually transmitted diseases.
They say in essence that they are not convinced that California should be the first jurisdiction in the country to limit liability for the negligent transmission of HIV only to those who have actual knowledge they are HIV positive.
The ruling has come about as a result of a lawsuit involving a married couple in which the wife and husband were both infected with HIV and each is charging the other with transmitting the disease.
The wife in the case is demanding evidence of her husband's sexual history to establish he had reason to know of his infection.
Among the information that may have to be revealed, under the ruling, are the dates and frequency of a man's unprotected sexual contact with other men and with intravenous drug users, other high-risk behavior, and any medical tests or symptoms that might have indicated the defendant or a partner was infected.
It will not be mandatory that the names of past sexual partners be disclosed although there was concern that the ruling would encourage plaintiffs to try to identify those partners.
The fear was also expressed that by restricting liability only to those who have actual knowledge they are infected could discourage those who fear they may be infected from getting tested.
Another concern was that the absence of distinctive symptoms before a positive HIV test, would allow courts and litigants to delve into matters of sexual privacy and could discourage voluntary disclosure of infection.
According to the wife's lawsuit, between the time of their engagement in late 1999 and their marriage in July 2000, the husband assured his wife he was healthy and monogamous and insisted they stop using condoms during intercourse.
The wife suffered exhaustion and fevers in September 2000 and tested positive for HIV a month later, while the husband tested HIV-positive later that month and was diagnosed with AIDS in 2001.
The wife insists she was told her for the first time in November 2001 that he had engaged in sex with men before their marriage and that he had tested negative for HIV in August 2000.
The court has ruled that the husband must disclose when and how often he had sex with men, because it might indicate whether he should have known he was infected.