Chronic Fatigue and Immune Dysfunction Syndrome (CFIDS) is a medical condition that leaves patients feeling constantly tired and weak. The condition is also called Systemic Exertion Intolerance Disease (SEID), because even mild exertions can make the fatigue worse. Other synonyms include Myalgic Encephalomyelitis (ME), Chronic Fatigue Syndrome (CFS), and Post-Viral Fatigue Syndrome (PVFS). Though the exact cause of CFIDS is still unknown, it is believed to be linked to viral infections, immune system dysfunction, and genetic predisposition. There are currently no cure for CFIDS, but symptom management and lifestyle changes can help improve quality of life for those who suffer from this challenging condition.