Myalgic encephalomyelitis​/​chronic fatigue syndrome (ME/CFS) is a disabling chronic illness. People with ME/CFS experience profound fatigue that does not go away with rest, as well as sleep issues and problems with memory or concentration. The hallmark symptom is post-exertional malaise (PEM), a worsening of the illness that can start immediately or hours to days after even minor physical or mental activity. This "crash" can last from hours or days to several months. Further common symptoms include dizziness or faintness when upright and pain.
The cause of the disease is unknown. ME/CFS often starts after an infection, such as mononucleosis and it can run in families. ME/CFS is associated with changes in the nervous and immune systems, as well as in energy production. Diagnosis is based on distinctive symptoms, and a differential diagnosis, because no diagnostic test such as a blood test or imaging is available.