Sourcing HIV-1 infection HIV-1 has a multitude of strain variants, but sexual transmission of HIV-1 is assumed to result from productive infection by only one virus particle. Knowing the genetics of the virus strains that are transmitted could be crucial for developing successful vaccine strategies. Using epidemiological and genetic data from 112 pairs of sexual partners, Villabona-Arenas et al. found that individuals with acute infections are more likely to transmit multiple founder virus strains. In a phylodynamic approach that integrated phylogenetic analysis of sequence data with simulation of a transmission chain, the authors showed that multiple variant transmission is doubled during the first 3 months of infection irrespective of whether transmission was heterosexual or by men who have sex with men. Science , this issue p. 103