Smalpox or Smallpox: How To spell?

The word Smalpox is misspelled against Smallpox, a noun meaning "A contagious, constitutional, febrile disease characterized by a peculiar eruption; variola. The cutaneous eruption is at first a collection of papules which become vesicles (first flat, subsequently umbilicated) and then pustules, and finally thick crusts which slough after a certain time, often leaving a pit, or scar."