What is chronic fatigue syndrome?

Chronic fatigue syndrome (CFS) is an illness characterized by prolonged, debilitating fatigue that does not improve with rest and worsens with physical or mental activity. It is also characterized by multiple nonspecific symptoms such as headaches, recurrent sore throats, muscle and joint pains, and memory and concentration difficulties.

Profound fatigue can come on suddenly or gradually and persists or recurs throughout the illness. Unlike the short-term disability of an illness such as the flu, by definition CFS symptoms must have lasted for at least six months. They often linger for years. The cause of CFS is unknown.

CFS also may be called myalgic encephalomyelitis (ME or ME/CFS or CFS/ME) or chronic fatigue and immune dysfunction syndrome (CFIDS).