Horrorblind Fever

A strange disease that is said to be inflicted by the malice of the Grey Men or other swamp spirits, Horrorblind Fever sometimes afflicts those who spend too much time in Mousillon's swamps. Lasting a good three days, the sufferer has only mild symptoms (like intermittent rashes or a head cold) until they are placed under great stress, whereupon there is a chance they will be struck temporarily blind. Horrorblind Fever can be gravely hazardous to anyone who experiences fear, panic, or even just excitement regularly, which makes it very dangerous for adventurers.

Source
pg. 26
 * Warhammer Fantasy RPG 2nd ED -- Barony of the Damned