Addison's Disease - according to Kenneth Chalmers (http://www.amazon.co.uk/Bela-Bartok-20th-Century-Composers/dp/0714847704/ref=sr_1_22?ie=UTF8&s=books&qid=1247740960&sr=8-22)
According to Wikipedia, Bartok married Márta Ziegler in 1909. They had a son in 1910. They then divorced and he married Ditta Pasztory, a piano student. She had his second son, Péter in 1924