Not a puzzle:
1. First smallpox epidemic struck Mexico City in 1520, killing many of its inhabitants (1/5 - 1/2).
2. From 1803-06, the Balmis expedition carried vaccination around the world.
3. Smallpox is credited with population recovery in the 18th century and often blamed for slow growth in 19th century Latin America.
4. The last case of smallpox in Mexico occurred February, 1951 (and, in the world, in Somalia, 1977).
5. Evidence on genetic immunity and smallpox is rarely examined--and weak.