Altarian encephalitis was the disease caused by a retrovirus which could lie dormant many years before becoming active and subjecting its host to a coma and delirium for days. Once active, the virus erased 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 T. Riker had been infected with Altarian encephalitis in an illusion created by the alien Barash. It should be noted that there is no confirmation outside of this illusion whether the disease actually exists. (TNG: "Future Imperfect")