Phthisis

Phthisis is a disease caused by long exposure to powerstone radiation. A large list of symptoms was given in The Thran, but most of those could just have been included by Yawgmoth so he could diagnose healthy enemies with the disease and exile them to the Caves of the Damned. Rotting spots on the body and mental disorders are certainly part of the disease. However, its status as an actual disease is debatable as Dyfed accuses Yawgmoth of making false claims to justify his power.

Trivia

 * Phthisis is also an archaic name of the real-world disease tuberculosis.
 * The name was revisited in the "nostalgia" set Time Spiral and used for the black removal card Phthisis.
 * Phthisis is presumably similar to real-life radiation poisoning.