What Is Chronic Fatigue Syndrome? Chronic fatigue syndrome also called systemic exertion intolerance disease (SEID) as well as ME/CFS — is a serious, complex disorder of the central nervous system that affects the entire body. People with this illness feel constant, overwhelming tiredness along with other disabling symptoms that are not due to any other diagnosable medical condition. Did you know that anywhere from 800,000 to 2.5 million Americans may have CFS? Many people with chronic fatigue syndrome find it difficult to complete daily activities they once took for granted. It is chronic (which means long term), but symptoms may come and go and vary in severity. People may even experience the symptoms but not realize they have chronic fatigue syndrome. According to experts, remission is possible, though it’s unclear what will lead to that happy occurrence.