Acute Phase: A short, sharp, and relatively severe course of a disease; not chronic.

Anti-inflammatory: Agents that reduce inflammation without directly antagonizing the agent that caused it.

Anti-depressants: Pharmaceutical agents used to treat clinical depression.

Anxiety Disorders: Also known as anxiety neurosis or anxiety reaction. A condition that can be caused by both psychologic and physiologic factors. It can take two general forms: (1) acute anxiety (panic disorder), marked by repeated occurrences of intense self-limited anxiety lasting usually a few minutes to an hour, or (2) chronic anxiety, characterized by less intense reactions of much longer duration (days, weeks, or months).

Bipolar Affective Disorder: A mood disorder that commonly begins with depression and is characterized by at least one period of elation sometime during the course of the illness.

Borrelia: A genus of bacteria with numerous species that cause disease in humans. The diseases associated with these organisms are typically relapsing fevers.

Candida albicans: A common saprophyte of the digestive tract and female urogenital tract. It does not ordinarily cause disease, but may do so following a disruption of bacterial flora of the body, or in patients with depressed immune systems.

Case Definition: In the example of CFS, a combination of symptoms, signs, and physiologic characteristics that serve to distinguish a case of chronic fatigue syndrome from other disease states.

Case-control Study: An epidemiologic study that examines selected patients who have a defined disease (cases) with persons without the disease (controls).

Chronic: Of long duration, denoting a disease of slow progress and long continuance.

Chronic Fatigue and Immune Dysfunction Syndrome (CFIDS): A synonym for chronic fatigue syndrome used by some patients and physicians. It should be stressed, however, that no immune dysfunction or aberration has been persuasively linked to chronic fatigue syndrome.

Coenzyme: A substance that enhances or is necessary for the action of enzymes. They are generally much smaller than enzymes themselves.

Connective Tissue Disorder: A variety of inflammatory diseases of connective tissue, the most common of which is rheumatoid arthritis. Much, if not all, of this disease is now attributed to autoimmune processes.

Connective Tissue: The supporting tissues of the body, such as tendons, ligaments, bone, and cartilage.

Control: A device used to verify or regulate a scientific experiment or study. A case-control study serves as a useful example. Since patients with a specific illness are examined for various characteristics, a group of healthy individuals who otherwise have as much in common with the patients as possible must be examined in parallel for the same characteristics.

Cross-sectional Study: In epidemiology, a study in which participants are examined at only a single time for characteristics of a disease.

Cytokine: Proteins manufactured by cells of various lineages that, when secreted, drive specific responses (e.g., proliferation, growth, or maturation) in other susceptible cells.

Cytomegalovirus (CMV): One of the eight known types of human herpesviruses,also known as human herpesvirus 5 (HHV-5). It belongs to the beta subfamily of herpesviruses. CMV can cause severe disease in patients with immune deficiency and in newborns when the virus is transmitted in utero.

Depression: A neurotic or psychotic condition marked by an inability to concentrate, insomnia, and feelings of dejection and guilt.

Enterovirus: A genus of RNA viruses with over 70 types identified in humans. They reproduce in the intestinal tract, and various members can cause a variety of human diseases, including poliomyelitis, aseptic meningitis, hepatitis, inflammatory heart disease, and rhinitis.

Epidemiology: The branch of medical science that deals with the incidence, distribution, and control of disease in a population.

Epstein-Barr Virus (EBV): One of the eight known types of human herpesviruses, also known as human herpesvirus 4 (HHV-4). It belongs to the gamma subfamily of herpesviruses. It commonly causes acute mononucleosis, and less commonly chronic mononucleosis. It some populations EBV is causally associated with life-threatening malignancies (Burkitt's lymphoma, nasopharyngial carcinoma).

Etiology: Causal association of a disease with an agent. The study of the cause of diseases.

Fibromyalgia: Also known as myofascial pain syndrome and fibromyositis. A group of common rheumatoid disorders (not involving the joints) characterized by achy pain, tenderness, and stiffness of muscles.

Herpesvirus: A family of large DNA viruses that infect a wide range of animal species. Eight distinct types have been associated with a variety of human diseases.

Human Herpesvirus 6: A virus of the herpesvirus beta-subfamily, discovered in 1985, that infects more than 95% of people by the age of 2 years. It has been causally associated with roseola, mononucleosis-like illness, inflammation of lymph glands. There is also suggestive evidence for a role in multiple sclerosis.

Idiopathic: Denoting a disease of unknown cause.

Imaging Tests: Any of a variety of methods for observing the internal anatomy of the body, ranging from simple x-rays to complex three-dimensional scanning techniques using nuclear magnetic resonance, positron emission, and other techniques.

Immune Globulin: A crude preparation of antibody molecules collected from pooled multiple blood donations, used as a means for passively transferring antimicrobial resistance to susceptible individuals.

Immune Suppressants: Agents that block or restrict the activity of one or more components of the immune system, usually leading to increased susceptibility to infectious disease.

Lymph Node: Secondary immune organs distributed at discrete locations throughout the body. These organs play a central role in the activation and trafficking of immune lymphocytes in the body.

Lymphocyte: Small white blood cells that are uniform in appearance, but very diverse in function. Collectively, they are responsible for antibody production, direct cell-mediated killing of virus-infected cells and tumor cells, and for the regulation of virtually every other component of the mammalian immune system.

Magnetic Resonance Imaging: The use of nuclear magnetic resonance of protons to produce cross-sectional proton density images of internal structures of the human body.

Malaise: A feeling of general discomfort or uneasiness, an out-of-sorts feeling, often the first indication of an infection or other disease.

Myalgic Encephalomyelitis: A synonym for chronic fatigue syndrome in common usage in the United Kingdom and Canada.

Natural Killer Cell (NK): A lymphocyte which, unlike other lymphocytes, does not require specific activation by foreign antigen. They are considered to play a "front line" role in controlling infection, curbing infection until a specific, coordinated immune response can be mounted.

Neurasthenia: Nervous exhaustion. A functional neurosis marked by intense nervous irritability and weakness.

Neuromyasthenia: Muscular weakness, usually of emotional origin.

Neuropsychiatric: Relating to organic and functional diseases of the nervous system.

Positron Emission Tomography (PET scan): Imaging technique that relies on the detection of gamma rays emitted from tissues after administration of a natural biochemical substance into which positron-emitting isotopes have been incorporated.

Radionuclide Scans: Any of a variety of medical imaging methods that rely on atomic isotopes that decay and emit radiation.

Retrovirus: A family of RNA viruses that have the unique characteristic of producing an enzyme that makes a DNA copy of its genetic informationfrom an RNA template (the opposite of what normally takes place). The most widely recognized of these viruses is HIV, the causative agent in AIDS. Another virus from this family (HTLV-1) has been associated with T cell leukemia. Initial reports of an association of an HTLV-II-like retrovirus with CFS could not be confirmed in subsequent studies.

Sentinel Surveillance: A monitoring method that employs a surrogate indicator for a public health problem, allowing estimation of the magnitude of the problem in the general population.

Single Photon Emission Computed Tomography (SPECT) Scan: An imaging technique that measures the emission of photons of a given energy from radioactive tracers introduced into the body. As with other forms of computer-assisted tomography, the technique produces a series of cross-sectional images of internal anatomy.

Systemic Lupus Erythematosus: An inflammatory disease of connective tissue occurring predominantly in women (90%). It is considered to be an autoimmune disease.

T Lymphocyte, T cell: The most common type of lymphocyte, itself divided into at least three subpopulations on the basis of function -- cytotoxic, or killer T lymphocytes, helper T lymphocytes, and suppressor T lymphocytes. T cells play a cardinal role in regulating the immune system.

Titer: The concentration of a substance in a solution, or the strength of such a substance detected by titration. In the current context, the term is most likely to refer to antibody titer, which is a measure of the concentration of specific antibodies to selected microbes that are circulating in an individual's bloodstream.

Vitamin: A group of organic micronutrients, present in minute quantities in natural foodstuffs, that are essential to normal metabolism.