Clarke's Disease (also know as Clark's Syndrome) was a human-based brain disorder. The illness was degenerative. It led to hallucinations, the inability to recognize family, pain and eventually death.
Henry Archer suffered from the disease and passed away from it in 2124. The final years of life involved great amounts of pain and starting in 2122 Archer not recognizing his son and wife . In 2154, Arik Soong told Jonathan Archer that the elder Archer may have been cured of the syndrome if genetic engineering has been legal. The Earth Starfleet captain discussed the illness with Enterprise's CMO, Phlox and wondered if Soong was correct. (ENT episodes: "Borderland", "Cold Station 12")