Corprus is a terrible, incurable disease created by Dagoth Ur as part of his plan to take over Vvardenfell and eventually all of Tamriel. It plays a critical role in the main storyline of The Elder Scrolls III: Morrowind and is a constant threat and topic of discussion in the game.
Those infected with Corprus generally go insane and become horrific monsters. Around the Ghostgate area, the player can find multiple types of Corprus-diseased monsters, including zombie-like creatures which were obviously once human or elven. These monsters are covered in bulbous growths, are inhumanly strong, and will attack the player on sight.
However, it seems that some of those who are affected with Corprus do not simply change into mindless zombies. While meeting with the Nerevarine, Vivec tells the player that it is only weaker members of Dagoth Ur's army(and presumably those who contract it unwillingly) who become mindless. This means that the various "Ash" creatures, who have magical powers, show unflinching devotion to Dagoth Ur, and notably sport horrifying but very distinct growths, are probably infected with Corprus. Ascended Sleepers and Ash Ghouls such as Dagoth Gares show a pattern of losing visible evidence of eyes and growing tentacles from their faces as the disease progresses, but they also become fearsomely powerful.
Notably, denizens of the Corprusarium are victims of Corprus but do not attack the player. Yagrum Bagarn, living in the Corprusarium, has lost his legs to Corprus, but otherwise lacks most of the tumorous appearance of other Corprus sufferers.
Relevance to the Nerevarine
The Nerevarine is prophesied to be immune to disease, which is one side effect of Corprus. Thus, in the process of proving that he or she is the Nerevarine, the player character of Morrowind must contract and then partially cure Corprus. In between contracting and curing it, the Nerevarine appears repellent to those around him or her.