Smallpox ravaged the people of Europe and the Americas in the early modern era. Why it was a catastrophic cause of death for American Indians that helped lead to severe depopulation, but a manageable cause among Europeans that allowed continued population growth, has puzzled scholars. Research on variola continued after smallpox eradication in 1977, prompted in part by the fear that aerosolized smallpox might be used in bioterrorism. That research updates factors that may have aggravated smallpox lethality in American Indians, giving new information about infectivity, the proportion of people who may have contracted smallpox, the burden on infants of mothers who had not had smallpox, and the toll for pregnant women. This essay reviews old and new hypotheses about why so many in the New World died from smallpox using recent smallpox research and older sources.