My theory - which is only a WAG - is that some strain of SIV or HIV was pandemic among the ancestors of European humans a very, very long time ago, which explains the 15% heterozygous prevalence of the CCR-5 Delta 32 deletion in Europeans better than smallpox or plague.
The immunity is hypothesized to have to do with the method used by the HIV viruses, or by several bacteria, and yet other viruses, to infect the host cell.
Smallpox, black plague, HIV and cholera, among others, use the same or very similar, pathway. I know that I am naturally immune to cholera but I'm not going to do an experiment to see if I am immune to HIV, or smallpox!
My theory - which is only a WAG - is that some strain of SIV or HIV was pandemic among the ancestors of European humans a very, very long time ago, which explains the 15% heterozygous prevalence of the CCR-5 Delta 32 deletion in Europeans better than smallpox or plague. I don't think that scenario produces the picture we have. Neither apparently do most scientists. Most people still don't have the virus, but if its ancestor had been in Europeans long ago, all people of European descent would have some form of it.