people with ME/CFS do not have “fatigue as their main symptom”; they have post-extertional fatigability accompanied by malaise as their main symptom (their voluntary muscles do not work properly and are exquisitely painful after exercise)

Many people confuse “chronic fatigue” (which is a symptom of many chronic conditions, including rheumatoid arthritis, fibromyalgia, or lupus) or “general tiredness” (which is lifestyle-related), with the specific illness “chronic fatigue syndrome” (ME/CFS). To clarify the differences, here are some of the major symptoms of ME/CFS:

- unexplained physical and mental fatigue for an extended period of time
- post-exertional malaise (meaning an inappropriate loss of physical and mental stamina and a worsening of symptoms after any effort)
- sleep dysfunction
- pain
- neurological/cognitive manifestations
- autonomic manifestations, such as orthostatic intolerance
- neuroendocrine symptoms, such as subnormal body temperature
- immune system changes, such as recurrent flu-like symptoms.

the disabling fatigue experienced by individuals with ME/CFS differs from that associated with other illnesses or everyday activity

The diagnostic criteria for myalgic encephalomyelitis (ME) and chronic fatigue syndrome (CFS) define two distinct clinical entities. Cognitive impairment and post-exertional “malaise” (a long-lasting aggravation of typical symptoms, e.g., muscle weakness and cognitive “brain fog”, after minor exertion) are obligatory for the diagnosis ME, while chronic fatigue is the only mandatory symptom for the diagnosis CFS.

Finally, individuals with severe pathological fatigue might experience states that are very different from what a healthy individual experiences when fatigued.

The neurological/cognitive symptoms are more characteristically variable than constant and often have a distinct fatiguing component to them. Especially common are cognitive ‘fog’ or confusion, slowed information processing speed, trouble with word retrieval and speaking or intermittent dyslexia, trouble with writing, reading, and mathematics, and short-term memory consolidation.