Clarke's Disease (also called Clarke's Syndrome) was a progressive, degenerative disorder of the Human brain. Symptoms included hallucinations and (in the terminal stage) severe pain. The disorder was ultimately fatal, and was incurable as of the 22nd century.
Henry Archer, the father of Jonathan Archer, died of this illness in 2124. Dr. Arik Soong, attempting to sell the benefits of genetic engineering to Archer, suggested that the science might have led to a cure, sparing the elder Archer years of misery. (ENT: "Borderland", "Cold Station 12")