ME/CFS myalgic encephalomyelitis is a severe multisystem disease whose main symptom is post-exertional malaise (PEM). PEM is a worsening of condition and an increase in symptoms after physical, mental, or emotional exertion.
In addition to post-exertional malaise, affected individuals suffer from symptoms of the autonomic nervous system such as heart palpitations, dizziness, lightheadedness, and blood pressure fluctuations. There are also immunological symptoms such as a severe feeling of illness, painful and swollen lymph nodes, sore throat, respiratory infections, and increased susceptibility to infection. Furthermore, many affected individuals suffer from severe pain such as muscle and joint pain and new-onset headaches. Additionally, there are muscle twitches and cramps, severe sleep disturbances, and neurocognitive symptoms such as difficulties with concentration, memory, and word-finding (often referred to as "brain fog"), as well as hypersensitivity to sensory stimuli.
Therefore, severely affected individuals often have to lie in darkened rooms and can only communicate with relatives in whispers.