In 2367, while exploring a cavern on Alpha Onias III, Commander William T. Riker was rendered unconscious by gases. While unconscious, neural scanners scanned Commander William T. Riker's brain. The scanners used elements of Riker's reality and constructed a holodeck simulation with those elements interspersed throughout, so that it felt real to him. What Riker wanted, the scanners made possible.
In one of these simulations, on the Alpha Onias expedition, Riker had been infected by a strain of Altarian encephalitis. This disease was caused by a retrovirus that incorporated its DNA directly into the host's cells in the brain. This disease could lie dormant for many years before becoming active and subjecting its host to a coma, delirium, and fever. 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 2283, the disease became active in Captain Riker, the commanding officer of the USS Enterprise-D. For ten days, he was experiencing the symptoms of the disease. One morning, the fever broke, whereupon Riker woke up from his coma. Memories of the last sixteen years were supposedly lost to Riker. Doctor Beverly Crusher recommended associational therapy as a treatment for Riker. This therapy would help Riker regain his memories. (TNG: "Future Imperfect")