Tzaraas Summary

What is tzaraas, the skin affliction discussed ad nauseam in chapters 13-14 of Leviticus?

A natural physical illness
This is the view of Ralbag, Meshech Chachma, Rabenu Bechaya and Abravanel, who all seem to think of tazaraas as something contagious. In an approach that might today get him expelled from yeshiva, Ralbag interprets tzaraas in keeping with the medical theories of his time. 

A spiritual affliction
Most everyone else says that tzaraas can't be a physical illness, noting that tzaraas examinations are forbidden  on shabbos and holidays, and that the laws do not apply to non-Jews, or to houses outside of Israel.

The original understanding (perhaps)

Some think of tzaraas as leprosy. This is because the LXX translated the word as lepra or "a disease which makes the skin scaly" [Lepros means "scales"] However, the symptoms of tzaraas described in the Bible do not match the symptoms of the disease we call leprosy today. Possibly, what we now recognize as a variety of different skin diseases, or symptoms of other diseases, may have once all been called tzaraas. Robert Alter translates tzaraas as skin blanch, noting that in almost every case tzaraas presents as a loss of pigment. Alter suggests this whiteness of skin and hair reminded people of corpses, likely making them worry that whatever impurity that caused (or was caused by) death was present on the afflicted person. This, Alter continues, is why the remedy for tzaraas is virtually identical to the remedy for corpse contamination.

The rabbinic understanding
In BT Erechin seven sins are said to cause tzaraas; in Leviticus Rabba six sins are named (there is some overlap.) At some point, the rabbinic imagination became fixated on the idea that tzaraas was caused by sins committed through speech such as slander and gossip. This notion likely developed because Miriam is afflicted with tzaraas immediately after she maligns her brother. Also, Moshe suffers a brief bout of tzaraas at the burning bush after he bad mouths the Jewish people by suggesting they will ignore the message of deliverance. In another rabbinic interpretation the sin of the snake in the Garden of Eden is also used to bolster this notion. Drawing on the Greek translation (or perhaps a forgotten etymology of tzaraas) these rabbis suggested that the snake was punished with scales for misusing his power of speech when he tempted Eve.

The cure 

The Torah says tzaraas is treated with a potion made from the blood of a bird, cedarwood, red string (or wool) and hyssop. Originally, this potion may have been thought to have natural properties that made it effective at driving off the source of the impurity; alternativly, the symbolic action theory of magic suggests that these items were chosen because of the psychological effect the potion's ingredients had (or were expected to have) on the patient. Thus, the Rabbis suggested "The metzorah, who talked derogatorily about others consistently to his friends is likened to birds, who chatter endlessly. In a similar vein, the one who speaks ill of others is haughty, holding himself or herself high above others and is likened to the tall cedar. To be healed, the metzorah must erase arrogance, making themselves lowly like a worm (the word tola'as also means worm) or a hyssop."* To this Samson Raphael Hirsch added the idea that one bird is killed to represent the symbolic death of the "wild" part of ourselves that caused the sin. Robert Alter, who holds that the potion and accompanying ritual were believed to have some technology behind them, adds that one of the two birds is sent away after being sprinkled with the potion for the purpose of carrying away the tzaraas-causing impurity. (Compare with the seh l'Azazel, or scapegoat)

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*Passage in quotes was lifted from Wikipedia to spare me the bother of re-inventing the wheel