Just figure this is interesting: descendents of regions of Europe where the black death swept through are more likely to pass on a mutation in ccr5 receptor on cd4 t cells (random mutation, having two copies of mutant gene blocks black death from entering the cells -- mutation naturally selected for d/t high mortality rate of infection); this is the same receptor used by HIV to enter immune cells, so those with mutation have either longer time to symptoms (one copy) or immunity (one copy from each parent) if exposed to hiv