Chronic Fatigue Syndrome (CFS), also known as Myalgic Encephalomyelitis (ME), is a serious, long-term illness affecting multiple body systems. It causes debilitating fatigue not relieved by rest and worsened by activity (post-exertional malaise or PEM), unrefreshing sleep, and cognitive difficulties ("brain fog"). Other symptoms can include pain, dizziness, sore throat, and tender lymph nodes.The exact cause is unknown, with research exploring roles for infections (like viruses), immune system dysfunction, and genetic factors.Diagnosing ME/CFS is challenging as there's no specific test. Doctors rely on symptom criteria and rule out other conditions like thyroid problems, sleep disorders, or depression through medical history, physical exam, and tests.