Grey Fever

The real tragedy of this terrible plague is that most people do not recognise it as a disease. The symptoms are nearly identical to madness. And so, these individuals are locked away in an asylum where they spread it to the other inmates. The disease then spreads to the doctors and nurses, and ravages the entire place until everyone is either dead or mad. Grey Fever is marked by a wasting of the victim’s brain, resulting in hallucinations, eventually dementia, and finally a deep coma. Those who might recover from this disease are often driven mad by it, or by their new surroundings if they are left in one of the Empire’s horrible asylums.

Source

 * Warhammer Fantasy RPG 2nd ED -- Tome of Corruption (pg. 48).