It's also curious how a variation of the CCR5 gene, CCR5-Δ32, which gives increased immunity to HIV, is thought to have been selected for by smallpox in Northern Europe. While giving increased immunity to HIV and smallpox, it makes the carriers more suspectible to serious versions of flavivirus infections (tick-borne encephalitis, West Nile virus). This might be a pretty bad thing in the age of climate change!
https://en.wikipedia.org/wiki/CCR5#CCR5-%CE%9432