Myalgic Encephalomyelitis / Chronic Fatigue Syndrome (ME/CFS) is a serious, long-term illness that affects many body systems. People with ME/CFS are often not able to do their usual activities. At times, ME/CFS may confine them to bed. People with ME/CFS have severe fatigue and sleep problems. ME/CFS may get worse after people with the illness try to do as much as they want or need to do. This symptom is known as post-exertional malaise (PEM). Other symptoms can include problems with thinking and concentrating, pain, and dizziness.
Researchers have not yet found what causes ME/CFS, and there are no specific laboratory tests to diagnose ME/CFS directly.

Source : US Centers for Disease Control and Prevention.

ME /CFS is estimated to affect around 0.2 – 0.4% of the general population.

Note: The worsening of symptoms after physical exertion - PEM, as described above - is a further puzzling aspect.

“Post-exertional malaise following normal activities is unique to ME/CFS and we do not understand the biology underlying this severe and harmful feature of the disease,”
Source :US NIH