Over at Caterina.net they're discussing the "secret history of microorganisms", as one reader put it, prompting me to name-drop some books which might illuminate and/or entertainingly confuse this subject.
I can personally recommend Jared Diamond's Guns, Germs and Steel which, among other things, talks in detail about the asymmetry between Eurasia's stock of infectious diseases and the Americas'. As bad as European epidemics could be, they were nothing next to the decimation wrought by Eurasian germs on Native America, for complicated reasons having to do with the role of animal husbandry in creating both new diseases and immunity to them.
At the confusing end of things, I recall greatly enjoying Thomas M. Disch's novel Camp Concentration, which uses as its starting point the old theory that syphilis is somehow connected with genius (an idea that I understand has been thoroughly discredited by now).
Some books which I haven't read but which are on my infinite to do list: The Demon in the Freezer by New Yorker writer Richard Preston (now why is Amazon so prominently featuring the large-print edition? hmm) and Judith Miller et al's Germs: Biological Weapons and America's Secret War are current non-fiction bestsellers about smallpox and biological warfare, respectively. Very spooky stuff!
And it seems like I ran across an interesting pop-sci book on syphilis not long ago but I can't find it for sure; maybe I saw an advance review of Pox: Genius, Madness, and the Mysteries of Syphilis by Deborah Hayden. (Among Amazon's recommended purchases to accompany this last title is clean underwear. Go figure.)