A Biblical skin disease. Typically translated as Leprosy, but its symptoms are in fact different from those of Leprosy. In Hebrew, צרעת.
Biblically, the major cause of the disease was gossip. The disease is mentioned in the context of ritual purity and impurity, which need not have physical causes. Besides, we're dealing with God here. God can do things like that. Symptoms are basically skin lesions, wherein the skin is paler than normal. Extremely localized baldness on the scalp or in the beard may also indicate Tzara'at. There are other factors to look for, but those are the major ones.
As a side note, clothing and houses can also develop Tzara'at. The clothing of gossippers may develop red or green spots. This is how you parents out there may determine if your kids are saying things they shouldn't. If they try to give you any crap about "grass stains" or "spilled Hi-C", I would be especially worried. The Biblical treatment for afflicted clothing is to burn it. This, thankfully, occurs while the clothing is not being worn by its owners.
Houses develop similar spots as do clothing. Such houses are scraped, presumably in case some wise-ass came by the night before with a paintbrush. Should this fail to remove the affliction, the house is demolished, presumably forcing its formerly happy occupants to live on the street.
Assuming Tzara'at affects a person, they go off to live in quarantine until it looks like they're doing better. They then are treated to a fairly unique curing ritual. Blood from one bird is sprinkled upon another bird, some wood, and red thread, and then the live bird is set free. The formerly affected then shaves and soaks his clothing and person in water. A week later (s)he sacrifices two lambs, a ewe, some flour, and some oil. The person then returns to normal life, and lives happily ever after, except for the occasional nightmare about birds.
It is interesting to note, though, that if the Bible were attempting to treat Tzara'at as an infectious disease, its methods of quarantine, burning infected clothing, and (eventually) demolishing infected houses were rather forward thinking for the times.