Fever is an increase in body temperature of at least 1 degree Fahrenheit that is symptomatic of an underlying illness in a child. Fever can be as low as 99 degrees Fahrenheit (37.2 degrees Celsius) but is more likely to be at least 100 degrees Fahrenheit (37.8 Celsius) in children.

Normally, the body maintains an internal temperature somewhere between 97 degrees and 100.5 degrees Fahrenheit (36.1 degrees and 38.1 degrees Celsius), with the average being 98.6 degrees Fahrenheit (37 degrees Celsius). The exact temperature varies from child to child, and a child’s body temperature also fluctuates throughout the day. Typically, this variation is only 1 degree or so, and body temperature tends to be lower in the morning and higher in the evening. 

Fever is part of the body’s response to disease. The portion of the center of the brain called the hypothalamus increases internal body temperature to kill germs and stimulate the immune system to fight bacteria and viruses. Various conditions can cause a fever to develop. In most cases, these sicknesses are minor, such as a mild bacterial or viral infection. However, sometimes fever is an indicator of a more serious disease, especially when the fever occurs in very young children.

Fevers are either acute (short duration with a definite endpoint) or chronic (recurring repeatedly over time). Because fever is a natural and healthy response to illness, it typically does not need to be treated except to relieve a child’s discomfort. However, fever can become dangerous if a child’s body temperature becomes too high.

A thermometer can be used to detect a fever. These instruments come in many varieties, including digital thermometers, electronic ear thermometers, forehead thermometers and pacifier thermometers. Digital thermometers provide the most quick and accurate temperature readings. Forehead and pacifier thermometers are less accurate and are usually not recommended.

In the past, mercury thermometers were the standard instrument used to take a child’s temperature. However, organizations such as the American Academy of Pediatrics no longer approve of the use of mercury thermometers because they can potentially expose a child to the toxic effects of mercury. Mercury thermometers are no longer commercially available.

Temperatures are most often taken with oral, rectal or axillary (under the arm) thermometers. In most cases, temperatures should be taken rectally in children younger than age 3 and orally thereafter. When taking a temperature rectally, the thermometer should be coated in petroleum jelly and inserted a half-inch (1.27 centimeters) into the rectum and held there until temperature is taken, usually about one minute.

The following standards are used to determine when a child has a fever: