"Virgin-soil death rates for smallpox are hard to establish because for the last century most potential research subjects have been vaccinated. But a study in the early 1960s of seven thousand unvaccinated smallpox cases in southern India found that the disease killed 43 percent of its victims. Noting the extreme vulnerability of Andean populations--they would not even have known to quarantine victims, as Europeans had--Dobyns hypothesized that the empire's population "may well have been halved during this epidemic." In about three years, that is, as many as one out of two people in Tawantinsuyu died. The human and social costs are beyond measure."