Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a disabling chronic illness. People with ME/CFS experience profound fatigue that does not go away with rest, sleep issues, and problems with memory or concentration. Further common symptoms include dizziness, nausea and pain. The hallmark symptom is a worsening of the illness which starts hours to days after minor physical or mental activity. This "crash" can last from hours or days to several months.