Fever is the temporary increase in the body's temperature in response to some disease or illness.

A child has a fever when the temperature is at or above one of these levels:

• 100.4 °F (38 °C) measured in the bottom (rectally)
• 99.5 °F(37.5 °C) measured in the mouth (orally)
• 99 °F (37.2 °C) measured under the arm (axillary)

An adult probably has a fever when the temperature is above 99 - 99.5 °F (37.2 - 37.5 °C), depending on the time of day.

See also:

• Heat emergencies
• Temperature measurement

Normal body temperature may change during any given day. It is usually highest in the evening. Other factors that may affect body temperature are:

• In the second part of a woman's menstrual cycle, her temperature may go up by 1 degree or more.
• Physical activity, strong emotion, eating, heavy clothing, medications, high room temperature, and high humidity can all increase your body temperature.

Fever is an important part of the body's defense against infection. Most bacteria and viruses that cause infections in people thrive best at 98.6 °F. Many infants and children develop high fevers with minor viral illnesses. Although a fever signals that a battle might be going on in the body, the fever is fighting for the person, not against.

Brain damage from a fever generally will not occur unless the fever is over 107.6 °F (42 °C). Untreated fevers caused by infection will seldom go over 105 °F unless the child is overdressed or trapped in a hot place.

Febrile seizures do occur in some children. However, most febrile seizures are over quickly, do not mean your child has epilepsy, and do not cause any permanent harm..

Unexplained fevers that continue for days or weeks are called fevers of undetermined origin (FUO).

Almost any infection can cause a fever. Some common infections are:

• Infections such as pneumonia, bone infections (osteomyelitis), appendicitis, tuberculosis, skin infections or cellulitis, and meningitis
• Respiratory infections such as colds or flu -like illnesses, sore throats, ear infections, sinus infections, infectious mononucleosis, and bronchitis
• Urinary tract infections
• Viral gastroenteritis and bacterial gastroenteritis

Children may have a low-grade fever for 1 or 2 days after some immunizations.

Teething may cause a slight increase in a child's temperature, but not higher than 100 °F.

Autoimmune or inflammatory disorders may also cause fevers. Some examples are:

• Arthritis or connective tissue illnesses such as rheumatoid arthritis and systemic lupus erythematosus
• Ulcerative colitis and Crohn's disease
• Vasculitis or periarteritis nodosa

The first symptom of a cancer may be a fever. This is especially true of Hodgkin's disease, non-Hodgkin's lymphoma, and leukemia.

Other possible causes of fever include:

• Blood clots or thrombophlebitis
• Medications, such as some antibiotics, antihistamines, and seizure medicines