Green’s Dictionary of Slang

smallpox n.

SE in slang uses

In phrases

have smallpox (v.)

(US Und.) to be wanted on an arrest warrant.

1940

D. Maurer Big Con 262: To have smallpox. To be wanted on a warrant; ‘smallpox’ is said to be ‘catching’ because anyone in the company of the wanted man may be arrested also.