The Center for Disease Control and Prevention (CDC) tells us that more than a million Americans suffer from (CFS.) It's a term I'm familiar with. My daughter has suffered from it for years. She also suffers from fibromyalgia, which, she tells me, is just CFS with pain.
Wikipedia explains it this way:
Chronic fatigue syndrome (CFS), also known as myalgic encephalomyelitis (ME), post-viral fatigue syndrome (PVFS), and various other names, is a syndrome (or group of syndromes) of unknown and possibly multiple etiologies, affecting the central nervous system (CNS), immune, and many other systems and organs.
Dr. Weil says:
Chronic fatigue is a common complaint in examination rooms worldwide. Causes are tough to tease apart: stress, depression, lack of exercise, poor diet and existing medical conditions such as Parkinson's disease, cancers, multiple sclerosis and more can all contribute to it.
Specifically, the scientists think the hypothalamus-pituitary-adrenal axis (HPA) is involved in the disorder. "This is a physiologic marker of accumulated adaptation to stress," Reeves said. "Our working hypothesis is that the HPA axis and the brain is a plastic organ which changes its actual physical architecture depending on stresses accumulated over the lifetime. To some extent, genetics determine how you react to these stressors and, more important, they actually determine your subsequent reaction to stress later during the life span."