There is one disease in all the repertoire of pathology that defies almost all magical attempts at curing. That is the dreaded disease of leprosy. It is the single disease that resists completely the spell cure disease. As such, it is especially dreaded, not only for its debilitating effects, but for the fact that once you’re a leper, you’re a leper for life. The spell heal will cure leprosy, but only if the subject fails a saving throw vs. magic. If the saving throw is successful, not only is the intended subject still a leper the one who attempted to cast the spell will also contract the disease automatically. Only one attempt to cast heal can be made by the same caster on the same leper. Deities who cast heal on a leper will be automatically successful in the attempt.
It is not a disease that can be contracted casually; it can only be contracted from other lepers or through certain special circumstances (some monsters, for example, transmit the disease, it could be caused by cursed scrolls or other items, and the game master may designate any other circumstances as he sees fit.
The chance of contracting leprosy from contact with a leper depends on the level of contact.
Level of contact
Chance to contract
There’s a leper within 10 miles*
1% of 1% (roll d%; if it’s 01, roll again; if it’s 01 again, you have contracted leprosy)
You are in the same room as a leper
You drink from the same cup or sleep in the same bed as a leper
You are within 5’ of a leper
You physically touch a leper
* Does not apply if a leper colony or leprosaria is in the area, which imposes quarantine conditions on those within.
Those afflicted with leprosy are so afflicted for life. There is a 10% chance that the disease will be fatal in 1d6 months. Those who are affected will heal wounds at only 10% of the normal rate, and will find that cure spells (cure light wounds, etc.) do not affect them. Those afflicted will lose 2 points of charisma per month, although charisma can never go lower than 1. For those for whom the disease is not fatal, charisma loss will take place over the course of 1d6 months. Charisma is lost permanently.
(Please note that the disease of leprosy is a real-life disease, and in the real world it does not necessarily conform to the behaviors or statistics presented above. The above is an approximation of popular medieval beliefs surrounding the disease, adapted for use in a fantasy role-playing game.)