For instance, the Karimui Basin of the New Guinea highlands, where I worked in the 1960s, was occupied by an isolated population of a few thousand people, suffering from the world’s highest incidence of leprosy – about 40 percent! Finally, small human populations are also susceptible to nonfatal infections against which we don’t develop immunity, with the result that the same person can become reinfected after recovering. That happens with hookworm and many other parasites.