Science: F. Black (1994) and Peter J. Bianchine & Thomas A. Russo (1995)
Black: “Smallpox was clearly more virulent in the Americas than in Europe but the causes for this remain an open question.”
Bianchine & Russo: “given comparable care when ill and knowledge regarding the potential for surviving the illness, the death rates for virgin-soil Indian populations and repeatedly exposed Caucasian populations for measles were similar.”