If you are curious about how scientists go about working out whether Bubonic Plague could have been responsible for the increase in the mutant form of the CCR-5 gene (known as CCR5-Δ32) then take a look at this paper ' Evaluating plague and smallpox as historical selective pressures for the CCR5-Δ32 HIV-resistance allele' by Galvani and Slatkin. They suggest that smallpox might be a more likely cause than bubonic plague. I think the paper is quite accessible and you can see how they use simple genetic models to back up their argument.
'No single smallpox pandemic was as devastating as the Black Death, but the cumulative toll of human life caused by smallpox constituted an even stronger selection pressure than the episodic decimation of bubonic plague.'
Relevant material previously on the blog:
The Great Chinatown Fire
Flagellation in German rock video
Plague as a Biological Weapon