Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) is a severe chronic disease that affects various body systems.

A hallmark feature of ME/CFS is post-exertional malaise (PEM), wherein symptoms worsen after physical or mental exertion (leading to a so-called crash). In severe cases, even minimal activities, such as taking a shower, walking a few steps, or even turning over in bed, can be sufficient to trigger a crash.

The illness is also characterized by a wide range of other symptoms that can shift over time and differ from person to person. Patients can suffer from persistent exhaustion, unrefreshing sleep, profound weakness, problems with thinking and concentration (brain fog), orthostatic intolerance, pain, and many other symptoms.

Approximately 75% of patients are women. However, ME/CFS can affect anyone, male, female, young and older people, as well as children. Most frequently, its onset occurs in adolescents or young adults (late teens to mid-thirties). Often, but not always, it follows an acute infection. Known triggers include the Epstein-Barr virus (infectious mononucleosis), the influenza virus (flue), and the coronaviruses leading to SARS and COVID-19.