However, paradoxically at same time, the evolution of drug-resistant HIV may have actually reduced the severity of the city's epidemic, saving many men from becoming infected, the study said.

UCLA biomathematics professor Sally Blower said the model enabled the researchers to reconstruct the epidemic's past and predict its future by calculating the evolution of several classes of drug-resistant HIV strains in San Francisco.

The researchers examined the rise of HIV strains resistant to the three major classes of drugs -- nucleosides, non-nucleosides and protease inhibitors from 1987 to 2007.

Researchers found complex waves of rising and falling single-, dual- and triple-class drug-resistant HIV strains over 20 years. The model predicts that resistance to nucleosides will decline substantially and protease resistance will fall slightly through 2012, and that resistance to non-nucleosides will rise over the next five years and then begin falling.

The findings are presented at the annual American Association for the Advancement of Science conference in Boston.