Fig. 1From: Views from Understanding Evolution: Parasites and Pathogens Take the LeapSimplified phylogeny of HIV showing three separate colonizations of human hosts (Hahn et al. 2000). However, more recent analyses (Van Heuverswyn et al. 2006) suggest that the HIV-1 subgroup O may have a more complicated story: a strain of SIV may have made the leap from chimps to gorillas and then from gorillas to humans, resulting in the O subgroupBack to article page