Fifteen hundred years after it crumbled, the Roman Empire has left Europe with a legacy that Julius Caesar & Co. never imagined: the populations of areas which were once under Roman control are significantly more biologically vulnerable to HIV. While a good number of people in Europe and Western Asia possess a gene called CCR5-Delta32 that provides resistance to HIV infection, the gene is conspicuously less common in places that were part of the Roman Empire. The reason? It may be that certain diseases spread by the conquering Romans (who brought cats, donkeys, and mosquitos with them) specifically wiped out people with the CCR5-Delta32 variant. Oops.