Fever happens when the body’s internal “thermostat” raises the body temperature above its normal level. This thermostat is found in a part of the brain called the hypothalamus. The hypothalamus knows what temperature your body should be (usually around 98.6°F/37°C) and will send messages to your body to keep it that way.
Most people’s body temperatures even change a little bit during the course of the day: It’s usually a little lower in the morning and a little higher in the evening and can vary as kids run around, play, and exercise.
Sometimes, though, the hypothalamus will “reset” the body to a higher temperature in response to an infection, illness, or some other cause. Why? Researchers believe turning up the heat is the body’s way of fighting the germs that cause infections and making the body a less comfortable place for them.
Causes of Fever
It’s important to remember that fever by itself is not an illness — it’s usually a symptom of an underlying problem.
Fevers have a few potential causes:
Infection: Most fevers are caused by infection or other illness. A fever helps the body fight infections by stimulating natural defense mechanisms.
Overdressing: Infants, especially newborns, may get fevers if they’re over bundled or in a hot environment because they don’t regulate their body temperature as well as older kids. However, because fevers in newborns can indicate a serious infection, even infants who are overdressed must be checked by a doctor if they have a fever.
Immunizations: Babies and kids sometimes get a low-grade fever after getting vaccinated.
Although teething may cause a slight rise in body temperature, it’s probably not the cause if a child’s temperature is higher than 100°F (37.8°C).
When Fever Is a Sign of Something Serious
In the past, doctors advised treating a fever on the basis of temperature alone. But now they recommend considering both the temperature and a child’s overall condition.
Kids whose temperatures are lower than 102°F (38.9°C) often don’t need medication unless they’re uncomfortable. There’s one important exception to this rule: If you have an infant 3 months or younger with a rectal temperature of 100.4°F (38°C) or higher, call your doctor or go to the emergency department immediately. Even a slight fever can be a sign of a potentially serious infection in very young infants.
If your child is between 3 months and 3 years old and has a fever of 102.2°F (39°C) or higher, call your doctor to see if he or she needs to see your child. For older kids, take behavior and activity level into account. Watching how your child behaves will give you a pretty good idea of whether a minor illness is the cause or if your child should be seen by a doctor.
The illness is probably not serious if your child:
- is still interested in playing
- is eating and drinking well
- is alert and smiling at you
- has a normal skin color
- looks well when his or her temperature comes down
And don’t worry too much about a child with a fever who doesn’t want to eat. This is very common with infections that cause fever. For kids who still drink and urinate (pee) normally, not eating as much as usual is OK.