Edit: This is true, and fun information, and everyone downvoting here is either a fucking sheep, or grinding an axe. End edit.

One of the original prventatives for smallpox, the type of variolation that reached Europe from Asia and Africa, was to crush old blisters into a powder, and sniff up a line of it.

Variolation is giving a small dose to allow the immune system time to get trained on it before it gets critical.