Adrenal insufficiency: common in CFS; characterized by fatigue, low blood pressure, and low production of stress hormones. May be a result or cause of the condition.

Chronic Epstein-Barr Virus CEBV): the chronic form of the virus that causes mononucleosis. Appears to be present in some cases, but not all, of CFS.

Chronic Fatigue Immune Dysfunction Syndrome (CFIDS) and Post-Viral Fatigue Syndrome (PVFS): two other terms for CFS.

Cytomegalovirus (CMV): a herpes virus like EBV that is implicated in some cases of CFS.

Fibromyalgia (FM): a condition characterized by disabling muscle pain and tenderness; often accompanies CFS.

Mixed Infection Syndrome (MIS): used to describe multiple, simultaneous and synergistic infections, such as viruses, candida albicans and parasites, that often occur together in CFS.

Myalgic Encephalomyletis (ME): CFS as it is known in Canada and the United Kingdom.