I'd suggest "Guns, Germs, and Steel" by Jared Diamond. As he explains, many of the diseases that tend to be the worst killers are those with a non-human reservoir host, because even if an outbreak burns itself out by killing nearly everyone exposed, the pathogens remain in the host species, ready to cross the species barrier later on. He also discusses the role of population densities and trade routes in epidemics and the reasons that some societies such as the early Americans were nearly wiped out when the Europeans arrived, while the Europeans MAY have picked up one disease in exchange (syphilis).