By THE ASSOCIATED PRESS

octors once assumed that patients' natural immunity would keep them from getting the AIDS virus more than once. But Swiss researchers are now reporting that a 38-year-old man acquired a second strain of H.I.V. through unprotected sex more than two years after he was first infected in 1998.

The researchers, who describe the case today in The New England Journal of Medicine, say that it may complicate the search for an AIDS vaccine and that it redoubles the importance of safe sex practices, even among partners who are both already infected.

One researcher, Dr. Bernard Hirschel of the University of Geneva, said the authors had been able to document the case because the patient was enrolled in an AIDS drug study to test early treatment of the virus.

The man was successfully treated for more than two years and was taken off the drugs after getting an experimental vaccine intended to bolster his immune system. A few months later, and weeks after he had unprotected sex with men, his virus level jumped. He was found to be infected with a different strain.

The patient then resumed taking the drugs and has responded well.

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