Altairian encephalitis

A retro-virus that incorporates its DNA directly into the cells of its host, lying dormant up to many years before becoming active and subjecting the host to a coma and delirium for days. This inactivity erases through synapse degradation all memory back to the point of infection -- according to the future scenario the alien Barash constructed with Commander Riker. Apparently Klingons are immune to this infection.