Sagbata
Before the arrival of Western medicine the priests of Sagbata wielded tremendous power. They used the threat of smallpox (A killer disease) as a way of keeping people faithful to the god and Sagbata's shrines had small spots painted on their outer walls as a reminder.
The priests had discovered a way to immunize themselves against the disease and so were able to spread it at will using dried and infected scabs. They were of course greatly feared.
Adapted from 'A guide to the Gods' by Richard Carlyon