"The Rot" was the nickname for a genetically transmitted disease which exclusively affected Selelvians. Symptoms included abdominal cramps, headaches, nausea drowsiness and lethargy. It usually did not strike until middle age, although approximately three to four percent got it in their youth. There was no vaccine or cure for the Rot as of 2365.
That year, Jaan Devin, a Selelvian friend of Wesley Crusher, became afflicted with the Rot, as had his parents before him. He unintentionally used his Selelvian power of suggestion, known as "The Knack," to prompt Wesley to focus all his efforts into finding a cure for the disease, to the detriment of everything else in his life. A Kreel named Aneel promised to cure Jaan of the Rot in exchange for access to the Enterprise's armory and, later, assistance in taking the bridge. After this, Aneel killed Jaan, reasoning that death cured everything. (TNG novel: Strike Zone)