is that the disease is not actually a proper disease, but something else with symptoms of a disease
As a reveal trope, spoilers
- The Grey Death of Deus Ex is in fact nanomachines, with the symptoms caused by immune system rejection. The player character is genetically engineered to accept them and is immune as a result
- The "Corprus" in Morrowind is Dagoth Ur's divine will changing the effected creature.