Why did the Black Death kill the weak?

I suppose, because most pathogens do. But this NY Times article about recent paleopathological work on plague victims is interesting:

Most of the bone defects that the researchers found can be caused by malnutrition, and the scientists suggest that the findings may show effects of starvation on immune function. It is known from contemporary chronicles that many survived the plague, and they, the authors write, were probably well fed and healthy enough to mount an effective immune response.
"Even something as clearly deadly as the Black Death is still selective," said Sharon N. DeWitte, a co-author of the study and an assistant professor of anthropology at the State University at Albany. The Black Death, she continued, is comparable in some ways to various emerging diseases of today like Ebola or SARS, and studying it "gives us some insight into who might be at highest risk for these new diseases."

I think it is smart for paleopathologists to hook into this emerging diseases theme -- we saw it earlier this month with the syphilis origins story also. There is truth in the idea -- some of the best evidence we have about the dispersal and evolution of really dread diseases of history comes from paleopathological analyses like these. I hope we see many more.