Having established that, in some people, HIV in the genital tract is distinct from that in the rest of the body, Satish and the research team wanted to know whether the genital tract viruses had undergone convergent evolution. Was there a so-called “signature of selection” in HIV derived from semen? Unsurprisingly, in people with lots of gene flow between the genital tract and the blood, there was no easy key to identifying virus from the semen. However, in people with little gene flow between these tissues, a few mutations were unusually common among semen-derived viruses.

Can these mutations be used to build a vaccine that targets the HIV strains most likely to be transmitted during sex? Finding them is certainly a step in the right direction. Satish is pursuing this possibility by focusing on the particular viruses involved in transmission. He and his colleagues are currently studying cases in which a recently infected individual (the recipient), as well as the partner that infected him or her (the donor), are willing to participate in a study. Satish will use virus samples and phylogenetics to reconstruct the likely genetic sequence of the common ancestor of all the recipients’ viruses and will compare this sequence to the viral diversity in the donor’s semen and blood. He hopes this reconstruction will clarify which genetic traits make a virus more transmissible — and hence, which kinds of viral particles it is most important for a vaccine to block.