Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a disease that causes someone to feel so ill that they can't do their normal daily activities. People with ME/CFS have problems sleeping and extreme fatigue that doesn't get better with rest. Any kind of activity often makes their symptoms worse. This is called post-exertional malaise. They may have other symptoms like problems with thinking.

Another name for this condition is systemic exertion intolerance disease (SEID).