Darnay's disease was a terminal illness with no known cure, which attacked the central nervous system and the brain. Among its symptoms were coughing, pain, shortness of breath, irritability, and erratic behavior.
In 2365, Doctor Ira Graves was diagnosed with the final stages of Darnay's disease. (TNG: "The Schizoid Man")
An early draft of the script called the disease "Varnay's disease".