Fever is the body's normal and healthy reaction to infection and other illnesses, both minor and serious. Fevers help the body fight infection. Fever is a symptom, not a disease. In most cases, fever means your child has a minor illness. Often you must look at your child's other symptoms to determine how serious the illness is. Although it may be scary when your child's temperature goes up, fever is not harmful.
Normal body temperature
The average normal body temperature is about 98.6°F (37°C). It usually rises during the day from a low of 97.4°F (36.3°C) in the morning to a high of 99.6°F (37.6°C) in the late afternoon. Each child has a normal temperature range that may be different from another child's. Mild increases to 100.4°F (38°C) can be caused by exercising, wearing too many clothes, taking a hot bath, or being outside in hot weather.
Temperature varies depending on how you take it. The most common ways to measure it are:
You can also use:
- Forehead thermometers.
- Pacifier thermometers.
If you think your child has a fever but you are not able to measure his or her temperature, it is important to look for other symptoms of illness.
Children tend to run higher fevers than adults. The degree of fever may not indicate how serious your child's illness is. With a minor illness, such as a cold, a child may have a temperature of 104°F (40°C), while a very serious infection may not cause a fever or may cause only a mild fever. With many illnesses, a fever temperature can go up and down very quickly and often, so be sure to look for other symptoms along with the fever.
Babies with a fever often have an infection caused by a virus, such as a cold or the flu. Infections caused by bacteria, such as a urinary infection or bacterial pneumonia, also can cause a fever. Babies younger than 3 months should be seen by a doctor anytime they have a fever because they can get extremely sick quickly.
A fever in a healthy child is usually not dangerous, especially if the child does not have other symptoms and the fever goes away in 3 to 4 days. Most children who have a fever will be fussy and play less and may not eat as much as usual.
High fevers may make your child uncomfortable, but they rarely cause serious problems. There is no medical evidence that fevers from infection cause brain damage. The body limits a fever caused by infection from rising above 106°F (41.1°C). But outside heat-such as from being in a car that is parked in the sun-can cause body temperature to rise above 107°F (41.7°C), and brain damage can occur.
Childhood immunizations can reduce the risk for fever-related illnesses, such as Haemophilus influenzae type bHaemophilus influenzae (Hib) infection. Although no vaccine is 100% effective, most routine childhood immunizations are effective for 85% to 95% of the children who receive them. For more information, see the topic Immunizations.
Causes of fever
It is not unusual for a preschool-aged child to have 7 to 10 viral infections in a year. Each new viral infection may cause a fever. It may seem that a fever is ongoing, but if 48 hours pass between fevers, then the new fever is most likely from a new illness.
Common causes of fever include:
- Viral infections, such as colds, flu, and chickenpox.
- Bacterial infections, such as a urinary tract infection.
Teething does not cause a fever. If a baby is teething and has a fever, look for other symptoms that may need to be evaluated.
A fever that increases quickly may lead to a fever seizure in some children. After a fever has reached a high temperature, the risk of a seizure is less. Fever seizures can be frightening to see, but they usually do not cause other problems, such as brain damage, intellectual disability, or learning problems. If your child has a high fever and a seizure, see the topic Fever Seizures.
Low body temperature
An abnormally low body temperature (hypothermia) can be serious, even life-threatening. Low body temperature may occur from cold exposure, shock, alcohol or drug use, or certain metabolic disorders, such as diabetes or hypothyroidism. A low body temperature may also be present with an infection, particularly in newborns, older adults, or people who are frail. An overwhelming infection, such as sepsis, may also cause an abnormally low body temperature.
Check your child's symptoms to decide if and when your child should see a doctor.