This: the medical condition

Fever (also known as pyrexia, from the Greek pyretos meaning fire, or a febrile response, from the Latin word febris, meaning fever, and archaically known as ague) is a frequent medical sign that describes an increase in internal body temperature to levels above normal. Fever is most accurately characterized as a temporary elevation in the body's thermoregulatory set-point, usually by about 1–2 °C.

Fever differs from hyperthermia. Hyperthermia is an increase in body temperature over the body's thermoregulatory set-point, due to excessive heat production or insufficient thermoregulation, or both. Carl Wunderlich discovered that fever is not a disease but a symptom of disease.

The elevation in thermoregulatory set-point means that the previous "normal body temperature" is considered hypothermic, and effector mechanisms kick in. The person who is developing the fever has a cold sensation, and an increase in heart rate, muscle tone and shivering attempt to counteract the perceived hypothermia, thereby reaching the new thermoregulatory set-point. A fever is one of the body's mechanisms to try to neutralize the perceived threat inside the body, be it bacterial or viral.

Measurement and normal variation

When a patient has or is suspected of having a fever, that person's body temperature is measured using a thermometer.

At a first glance, fever is present if:

• Temperature in the anus (rectum/rectal) or in the ear (otic) is at or over 38.0°C (100.4°F)
• Temperature in the mouth (oral) is at or over 37.5 °C (99.5 °F)
• Temperature under the arm (axillary) is at or over 37.2 °C (99.0 °F)

The common oral measurement of normal human body temperature is 36.8±0.7 °C (98.2±1.3 °F). This means that any oral temperature between 36.1 and 37.5 °C (96.9 and 99.5 °F) is likely to be normal.

However, there are many variations in normal body temperature, and this needs to be considered when measuring for fever. The values given are for an otherwise healthy, non-fasting adult, dressed comfortably, indoors, in a room that is kept at a normal room temperature (22.7 to 24.4°C or 73 to 76 °F ) , during the morning, but not shortly after arising from sleep. Furthermore, for oral temperatures, the subject must not have eaten, drunk, or smoked anything in at least the previous fifteen to twenty minutes.

Body temperature normally fluctuates over the day, with the lowest levels around 4 a.m. and the highest around 6 p.m. (assuming the subject follow the prevalent pattern, i.e, sleeping at nighttime and staying awake during daytime). Therefore, an oral temperature of 37.2 °C (99.0 °F) would strictly be a fever in the morning, but not in the afternoon. An oral body temperature reading up to 37.5 °C (99.5 °F) in the early/late afternoon or early/late evening also wouldn't be a fever. Normal body temperature may differ as much as 1.0 °F between individuals or from day to day. In women, temperature differs at various points in the menstrual cycle, and this can be used for family planning (although temperature is only one of the variables). Temperature is increased after eating, and psychological factors also influence body temperature.