Jail fever: Epidemic typhus, a severe acute (sudden-onset) infectious disease with prolonged high fever up to 40° C (104° F), intractable headache, and a pink-to-red raised rash. The cause is a microorganism called Rickettsia prowazekii. It is found worldwide and is transmitted by lice. The lice become infected on typhus patients and transmit illness to other people. The mortality increases with age and over half of untreated persons age 50 or more die. Also called European, classic, or louse-borne typhus.

Jakob-Creutzfeldt disease: A transmissible degenerative brain disorder technically termed spongiform encephalopathy. Eating "mad cow" meat or squirrel brain can lead to Jaqcob-Creuzfeldt-like disease. Jakob-Creutzfeldt disease, better known as Creutzfeldt-Jakob disease (CJD), a dementing disease of the brain. It is believed due to an unconventional, transmissible agent (a prion). Symptoms of CJD include forgetfulness, nervousness, jerky trembling hand movements, unsteady gait, muscle spasms, chronic dementia, balance disorder, and loss of facial expression. CJD is classified as a spongiform encephalopathy. Most cases occur randomly (sporadically), but inherited forms exist. There is neither treatment nor cure for CJD. Other names for CJD include Creutzfeldt-Jakob syndrome, Jakob’s disease, and spastic pseuodoparalysis.

Jakob’s disease: Better known as Creutzfeldt-Jakob disease (CJD), a dementing disease of the brain. It is believed due to an unconventional, transmissible agent (a prion). Symptoms of CJD include forgetfulness, nervousness, jerky trembling hand movements, unsteady gait, muscle spasms, chronic dementia, balance disorder, and loss of facial expression. CJD is classified as a spongiform encephalopathy. Most cases occur randomly (sporadically), but inherited forms exist. There is neither treatment nor cure for CJD. Other names for CJD include Creutzfeldt-Jakob syndrome, Jakob-Creutzfeldt disease, and spastic pseuodoparalysis.

jamais vu: From the French, meaning "never seen". The illusion that the familiar does not seem familiar. The opposite of the feeling of "déjà vu."

Jaundice: Jaundice is a yellowish staining of the skin and white of the eyes (sclerae) with pigment of bile. Jaundice can be an indicator of liver or gallbladder disease or result from red blood cells rupturing (hemolysis).

Jaw: The bones below the mouth (the mandible) and the bone above the mouth just above the mouth (the maxilla). The word jaw came from the Anglo-Saxon ceowan meaning to chew.

Jejunal: Having to do with the jejunum.

Jejunum: Part of the small intestine. It is half-way down the small intestine between its duodenum and ileum sections.

Joint: A joint is the area where two bones are attached for the purpose of motion of body parts.

Joint hypermobility syndrome: A common benign childhood condition involving hypermobile joints (that can move beyond the normal range of motion). Symptoms include pains in knees, fingers, hips, and elbows. The affected joints may sprain or dislocate. Scoliosis (curvature of the spine) is more frequent. Usually improves with adulthood. Also called the hypermobility syndrome.

Joint, tempero-mandibular: Joint that hinges the lower jaw (mandible) to the skull. Abbreviated TMJ or TM joint.

Joint, TM: The joint that hinges the lower jaw (mandible) to the skull.

Jugular: The principal vein in the front of either side of the neck. The word comes from the Latin jugulum meaning throat. The jugular is "the vein of the throat" or in ancient times "the sacrificial vein."

Juvenile: Between infantile and adult as, for example, in juvenile rheumatoid arthritis (onset before age 16 years).

Juvenile chronic arthritis, systemic-onset: See: Juvenile rheumatoid arthritis, systemic-onset (Still’s disease).

Juvenile rheumatoid arthritis, systemic-onset (Still’s disease): Also known as systemic-onset juvenile chronic arthritis. Still’s disease presents with systemic (bodywide) illness including high intermittent fever, a salmon-colored skin rash, swollen lymph glands, enlargement of the liver and spleen, and inflammation of the lungs (pleuritis) and around the heart (pericarditis). The arthritis may not be immediately apparent but it does always surface and it may persists long after the