Herpes

Herpes is an STD that has made two appearances on the series.

Old Frita
Old Frita had the disease in the episode Chickenpox. Stan finds out that chickenpox is actually a form of herpes, and  the boys get revenge on their parents for deliberately making them contract the virus.

Kenny's Herpes
Kenny had contracted the disease in the episode I Should Have Never Gone Ziplining. Throughout the episode, Cartman made fun of him for having the disease, though Kenny claimed that is was only a cold sore. Kenny later died, but surprisingly, not from his herpes.