By Jessa Forte Netting
DISCOVER Vol.26 No. 01
January 2005

Although several major HIV strains exist, only one type predominates in the United States,
leading infected Americans to assume they cannot be reinfected through unprotected sex with one
another. But a study of recent infections, published in September in the Journal of the
American Medical Association, shows reinfection, called superinfection, with another kind of
HIV can and does occur. "What we found was that you could get it again. And when you did, you
got it worse," says study author Davey Smith of the University of California at San Diego.

Smith and his colleagues analyzed stored blood samples from 78 people newly infected with HIV
and then compared these with blood drawn from the same people six months to a year later. Their
analysis revealed a second, genetically distinct virus proliferating in the blood of three
subjects-not a different strain but different enough to suggest a second infection. Worse,
these patients began to deteriorate. Within six months the virus load in their blood shot up
and their CD4 counts-a measure of immune system function-fell. The presence of the competing
viruses diminished the effectiveness of antiretroviral drugs.

An ongoing study presented in July at the International AIDS Conference in Bangkok provided an
even more chilling picture in Tanzania, where many HIV types coexist. Following 600
HIV-positive women, Francine McCutchan, of the U.S.-based Henry M. Jackson Foundation, has so
far found 28 percent are infected by more than one major HIV strain. There are fears-but as yet
no evidence-that such a mix of HIV could recombine in a person to produce a more virulent
strain.

In another report at the Bangkok conference, Robert Grant of the Gladstone Institute at the
University of California at San Francisco found no competing viruses among 28 couples and 31
individuals with long-standing HIV infections who have unprotected sex with HIV-positive
partners. All the subjects had been HIV-positive for more than a year. A second virus may be
able to take hold only shortly after the first HIV infection occurs. Smith cautions, "We are
dealing with a new weapon of mass destruction here."