Chronic fatigue syndrome (CFS)— a complex disorder characterized by extreme fatigue that lasts six months or longer, and does not improve with rest or is worsened by physical or mental activity.
M.E. (Myalgic Encephalomyelitis/ Encephalopathy) is a chronic, fluctuating illness. It is also known as Chronic Fatigue Syndrome (CFS). Sometimes it is diagnosed as Post Viral Fatigue Syndrome (PVFS). The illness affects many parts of the body, such as the nervous and immune systems. The most common symptoms are severe fatigue or exhaustion, problems with memory and concentration and muscle pain.
(Defintion taken from “Action for M.E” the UK's leading charity dedicated to improving the lives of people with M.E.)