Chronic fatigue syndrome (CFS) is a complicated disorder characterised by extreme fatigue that doesn't improve with bed rest and may worsen with physical or mental activity. Chronic fatigue syndrome may occur after an infection, such as a cold or viral illness. The onset can also be during or shortly after a time of great stress, or it may come on gradually without a clear starting point or obvious cause. This book discusses new research in CFS, including the effects that CFS may have on immune system responses. The possible implications and prevention of CFS are also explored.