1) General medical journal 
An illness that has persisted for a long period of time. It is a continuing disease process. ... (12 Dec 1998) ... 
Chronic illness: An illness that persists for a long period of time. The term 'chronic' comes from the Greek chronos, time and means lasting a long time. A chronic illness is one lasting 3 months or more, by the definition of the U.S. National Center for Health Statistics. In ancient Greece, the 'father of medicine' Hippocrates distinguished diseas...