Pox

google

ref

late Middle English: alteration of pocks, plural of pock.

wiktionary

ref

From the plural of Middle English pocke. See also pock.

etymonline

ref

pox (n.)

"disease characterized by eruptive sores," late 15c., spelling alteration of pockes (late 13c. in this sense), plural of pocke "pustule" (see pock (n.)). Especially (after c. 1500) of syphilis.