We are a relatively young kind of primate, we humans, and therefore our diseases are young too. We have borrowed our troubles from other creatures. Some of those infections, such as Hendra and Ebola, visit us only occasionally and, when it happens, arrive soon at dead ends. Others do as the influenzas and the HIVs have done—take hold, spread from person to person, and achieve vast, far-flung, enduring success within the universe of habitat that is us.
