The most devastating disease and the answer to his question was smallpox, but other deadly diseases included typhus, measles, influenza, bubonic plague, mumps, yellow fever and pertussis.
Many European emigrants who arrived after the outbreaks had already killed massive numbers of natives assumed that the natives had always been small in number. The scale of the epidemics over the years has been enormous, killing millions of people - about 90% of the population in the hardest hit areas - and creating "the greatest human catastrophe in history, probably even exceeding the Black Death disaster that killed a third of population between 1347 and 1351.