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. This study found a genetic component to ME/CFS in families with more than one generation of illness. Those families had significantly higher frequencies of maternal haplogroup subclades known for reduced OXHPOS capacity and mitochondrial uncoupling. Individuals with mitochondrial dysfunction can be pushed over the threshold into ME/CFS by viruses (e.g. Epstein Barr), infections (e.g. Lyme Disease), Gulf War Syndrome, and chemical exposure.