This study looks at the role of semen in promoting HIV-1 transmission during vaginal intercourse beyond its role as a passive carrier of virus. Semen of all healthy men contains high concentrations of interleukin-7 (IL-7), and concentrations are increased further in HIV-1-infected individuals. The authors used an ex vivo model of HIV-1 infection of human cervico-vaginal and lymphoid (tonsillar) tissue to show that short periods of exposure to concentrations of IL-7 comparable to those found in the semen of HIV+ men enhance subsequent HIV-1 replication by preventing the apoptosis and promoting the proliferation of CD4+ T cells. This increases the size and lifespan of the founder pool of infected cells. The authors suggest that the concentration of IL-7 in semen could be a determinant of the efficacy of HIV-1 transmission to a female partner.