And you were probably thinking that people only got crabs from coeds on spring break in Cancun!
Seriously though, I apologize for the sensationalist title, but this story is a really neat demonstration of the application of phylogenetic comparison and evolutionary divergence time estimation. (For a primer of phylogenetics see this post.)
The venereal affliction known as “crabs” is an infestation of lice that specifically reside in the pubic region of humans. Pubic lice, Pthirus pubis, are of a genetically and morphologically distinct genera of lice from the head louse, Pediculus humanus. If you compare the evolutionary history of primates with the history of the lice that parasitize them, an interesting picture emerges.
Phylogenetic cladograms, assembled from morphological and genetic characters, for primates and parasitic lice. Dotted lines indicate parasite-host relationships. Figure from Weiss, 2009; after Reed et al., 2007.