Altarian encephalitis
From Memory Alpha, the free Star Trek reference.
Altarian encephalitis is the disease caused by a retrovirus which can lie dormant many years before becoming active and subjecting its host to a coma and delirium for days. Once active, the virus erases all memories back to the point of infection through synapse degradation. Klingons seem to be immune to the infection.
In 2367, Doctor Beverly Crusher claimed that Commander William Riker had been infected with Altarian encephalitis in an illusion created by the alien Barash. (TNG: "Future Imperfect")
