"Pox" are spots, usually ulcerating or vesicular.
The word is used in various disease names, including names for:
- Syphilis - "the pox", "French pox", or "the great pox"
- Smallpox - presumably (rather oddly) not considered as great a plague as syphilis.
- Chickenpox (aka varicella) - presumably considered trivial (?chicken-shit) in comparison to the great pox.