New York Times, 5/26/98, B9. To put it simply, the biological mechanism underlying both diseases depends on the invading organisms' ability to suppress its victims' white blood cells, part of the human body's immunological response to infection. That is, both the AIDS virus and Yersinia pestis attack the same site in the immune system's defense network, the part which mounts a counter-attack to infection. Those individuals in the fourteenth century who had quick and effective immunological responses to Plague and thus survived the Black Death, were apparently able to hand on that genetic capability to their descendants. And this advantage, as it turns out, works not only against Yersinia pestis but all types of infections, including AIDS, which attempt to subvert the human body's immunological response to infection in the same way.

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