A pox is an illness, especially one that's particularly contagious and causes blisters or rashes. Today, most poxes are preventable through vaccines.
The most devastating of the poxes was probably smallpox, a viral disease that killed around 500 million people during the 20th century before being officially eradicated in 1980. A more common pox is chickenpox, which is highly infectious and can be fatal but is not as deadly. What these illnesses have in common is scarring blisters, once known themselves as pox, from pock, "pustule or blister."