Health

other primates are susceptible to smallpox, albeit lab-challenge does not typically cause the same spectrum of illness in these non-human primates (NHP) as in man. These NHP models were continually tested against anti-viral drugs, especially after bioterrorism fears peaked after 911. Prairie dogs and other non-primates are likewise susceptible to smallpox (and of course, monkeypox), but are difficult to study in lab conditions and considered unsuitable for strict biocontainment protocols.

So, the issue is no naturally-occurring non-human animal host for smallpox, making eradication possible. By contrast, monkeypox has a lot of naturally occurring animal hosts.

Smallpox is not the ONLY virus with only human hosts in nature. Measles, though unrelated to smallpox and other (Orthopox), likewise is on the possible list for eradication or at least elimination (current status in the Americas) because it has no naturally-occurring non-human hosts in animal populations.