Myalgic Encephalopathy or Myalgic Encephalomyelitis (ME), also known as Chronic Fatigue Syndrome (CFS), Post-Viral Fatigue Syndrome (PVFS) are several names given to a poorly understood, highly debilitating disorder of uncertain cause/causes, which is thought to affect approximately 4 per 1,000 adults in the United States and other countries, and a smaller fraction of children.

The disorder is marked by chronic mental and physical exhaustion, often severe, and by other specific symptoms, arising in previously healthy and active persons. Despite promising avenues of research, there remains no objective pathological finding which is widely accepted to be diagnostic of CFS/ME. It remains largely a diagnosis of exclusion, made on the basis of patient history and symptomatic criteria, although a number of tests exist which can help aid diagnosis.

Although there is agreement on the genuine threat to health, happiness, and productivity posed by CFS/ME, various physicians' groups, researchers, and patient activists champion very different ideas regarding diagnostic criteria and favored treatments, resulting in ongoing controversy about nearly all aspects of the disorder. The name chronic fatigue syndrome is itself controversial, with some patient advocates and other authorities preferring terms such as myalgic encephalomyelitis ("ME" or "CFS/ME") and post-viral fatigue syndrome ("PVFS"), which imply specific underlying etiologies or pathologic processes.

While fatigue is a common symptom in many illnesses, CFS/ME is a multi-symptom disease and is relatively rare by comparison. Definitions require a number of features, the most common being severe mental and physical exhaustion which is "unrelieved by rest" and may be worsened by even trivial exertion. Most diagnostic criteria insist that the symptoms must be present for at least six months, and all insist on there being no other cause for them: i.e. the symptoms must not caused by other medical conditions such as diabetes, hypothyroidism or anemia. Click here for more information about CFS/ME symptoms.

It is unclear if these symptoms represent co-morbid conditions or are produced by the same underlying etiology as CFS itself. Some cases improve over time, and treatments (though none are universally accepted) bring a degree of improvement to many others, though resolution is rare.

CFS/ME occurs more often, but not exclusively, in women, possibly due to immunological factors or hormonal changes. CFS/ME is most easily diagnosed when formerly active adults become ill, and is most commonly diagnosed in young to middle aged adults, although it is also reported in children, adolescents and the elderly.

Research suggests that CFS/ME results from a dysfunction of the immune system. The exact nature of this dysfunction is not yet defined, but generally it can be viewed as an over active state. For a period of time it was thought that Epstein Barr Virus, which causes Mononucleosis, was the cause of this illness. However, researchers believe that the Epstein Barr Virus, when it exists, is a result or complication of CFS/ME rather than its cause.

A lack of information and awareness has led to many patients being stigmatized as hypochondriac or lazy. The Centers for Disease Control & Prevention have now recognized CFS/ME as a serious illness and have recently launched a campaign to raise public and medical awareness about it. The American Medical Association, the World Health Organization, and the Nation