I am running a D&D 3.5 campaign, and a player wants to go for the Cancer Mage prestige class in the Book of Vile Darkness. Is this prestige class overpowered, as defined in this question?
Cancer Mage is one of the most notoriously-overpowered classes in the game
Only the illithid savant and beholder mage immediately come to mind as more powerful. The ability to ignore the negative aspects of diseases, but only the negative aspects, is extremely powerful, because many diseases do have some kind of “upside” (to make diseased creatures more dangerous, I suppose). Worse, several of these have cumulative upsides: something it keeps improving (until it kills you).
In particular, Festering Anger is supposed to keep improving your Strength until it kills you – except it will never kill a cancer mage. So the cancer mage just gets arbitrarily-high Strength. Vile Rigidity is another one, that keeps improving your natural armor. Note that both of these diseases are in Book of Vile Darkness right alongside the cancer mage, so... I suppose someone wasn’t paying attention.
That said, without these and other abusive diseases, the cancer mage is probably OK. Drawing the line between what is an abusive disease and what isn’t is tricky though.