The majority of people with ME/CFS report an infection or flu-like symptoms upon onset of their illness, but other sudden, precipitating events have also been reported. Onset may also be gradual without experiencing a notable precipitating event of any kind. The exact cause of this multi-system illness has not been definitively established, but research supports involvement of the central, peripheral, and autonomic nervous systems, aberrant immune activation and dysfunction, and abnormal and impaired cellular metabolism.
Patients experience a substantial reduction in their daily functioning and can experience a broad range of symptoms including profound fatigue, post-exertional malaise, unrefreshing and disordered sleep, cognitive impairment, orthostatic intolerance, flu-like symptoms, sensory sensitivity, amplified pain, recurrent headaches, and gastrointestinal disturbances.
The hallmark and defining feature of the illness is “post-exertional malaise” or PEM. Patients experiencing PEM will often describe a “crash,” “relapse,” or “collapse” after even small amounts of mental or physical exertion that was previously tolerated. During the crash, which may be immediate but more often delayed by hours or days, patients can experience an exacerbation of one or all of their symptoms and a further reduction in functioning. It can take hours, days, a week or even longer to return to their previous baseline after a crash. Some patients may go through cycles of overexerting and crashing while others may have learned to reduce or change activities to minimize crashes. For some patients, even basic activities of daily living can result in PEM.