The 4-3 ruling was the state Supreme Court's first involving allegations of negligent HIV infection, and makes those with "constructive knowledge" — people who should have known by their behavior and other signs they were infected but perhaps did not — legally liable for infecting others.
The case involved a woman who accused her husband of infecting her with HIV during their honeymoon.
Writing for the majority, Justice Marvin Baxter said that "society has an overriding policy of preventing the spread of sexually transmitted diseases, especially HIV, which would be enhanced by imposing a duty of care on those who have reason to know they are infected with HIV."
Baxter also wrote that "negligent transmission of HIV does not depend solely on actual knowledge of HIV infection and would extend at least to those situations where the actor, under the totality of the circumstances, has reason to know of the infection."
A federal court in Michigan is the only other jurisdiction to rule similarly in a 1993 case involving former NBA star Earvin "Magic" Johnson, who tested positive for HIV in 1991.
Three of the California high court's seven judges each wrote separate dissents, including Justices Kathryn Mickle Werdegar and Carlos Moreno, who said it was unfair to punish those who "had reason to know" they had HIV but didn't know for sure.
"I agree that a defendant who knows that he or she is infected with HIV and conceals that fact from a partner with whom the defendant has unprotected sex may be held liable for negligently transmitting the virus," Moreno wrote.
Moreno said he feared that such a ruling will dissuade people from taking HIV tests and also foster meritless lawsuits.
The 4-year-old case was closely watched because the emotional distress and fraud lawsuit filed by the woman identified only as Bridget B. exposed a whole raft of legal issues dealing with privacy rights versus how much information about a partner's sexual past is legally required to be disclosed.
But the court's ruling did not give Bridget B.'s case much of a boost. It limited her access to her ex-husband's sexual history and medical records to just six months from August 2000, when he tested negative for HIV as part of a medical checkup required for purchasing a life insurance policy.
The ex-wife had demanded the names and address of all of John B.'s homosexual partners in the 10 years preceding their July 2000 wedding.
Her attorney didn't return a telephone call Monday.
The couple met in 1998 and married in 2000 and last had intercourse during the honeymoon in July 2000. In September 2000, she began suffering from exhaustion and fevers and tested positive for HIV the following month. John tested positive shortly after and his doctor told Bridget that she had "brought the HIV into the marriage."
Court documents show, however, that she learned in November 2001 that she had an extremely low probability of bringing AIDS into the marriage. The next month, John told Bridget for the first time that he previously had sex with men. She sued him in 2002 and he countersued, accusing her of infecting him.
"This is a sad case," Baxter wrote for the majority court.