Fever, known medically as pyrexia, is an elevation of the body temperature above the normal 37°C (98.6°F). Fever is mostly caused by viral or bacterial infection and can accompany virtually any infectious disease. Inflammation, heat stroke, and some tumors are also important causes. Fever due to infection is produced by proteins called pyrogens, which are derived from cell products, and alter the set level of temperature-regulating centers in the hypothalamus in the brain. Fever may be continuous, intermittent or remittent, the distinction helping to determine the cause. Anti-inflammatory drugs (e.g. aspirin) reduce fever; steroids mask it. The highest point of a fever is called the fastigium.


A fever may be accompanied by other symptoms, such as shivering, headache, sweating, thirst, a flushed face, hot skin, and faster than normal breathing. In some cases there may be rigors (attacks of severe shivering followed by drenching sweats and a sudden fall in body temperature). Confusion or delirium sometimes occurs with fever, especially in the elderly; a very high fever may also cause seizures or coma, especially in children.



Most fevers are caused by viral or bacterial infections, such as influenza, measles, or tonsillitis. In these cases, pyrogens are released when white blood cells (leukocytes) of the body's defense system fight the microorganisms responsible for the illness. These proteins act on the temperature-controlling center in the hypothalamus, causing it to raise body temperature in an attempt to destroy the invading microbes.


Fever may also occur in noninfectious conditions, such as dehydration, thyrotoxicosis (a condition that results from overactivity of the thyroid gland, heart attack, and lymphoma (a tumor of the lymphatic system).



A doctor should be consulted if a fever lasts more than three days or if there are worrying accompanying symptoms, such as a severe headache, with stiff neck, abdominal pain, or pain when passing urine. Prompt medical advice is also necessary if fever occurs in a baby who less than six months old, in a child with a history of febrile convulsions, or in an elderly person.


Antipyretic (fever-reducing) drugs, such as aspirin (adults only) and paracetamol, may be given to treat fevers due to infections; such drugs also help relieve any aches and pains accompanying the fever. Otherwise, treatment is directed toward the underlying cause (for example, giving antibiotics for a bacterial infection).