Fevers 101: What you need to know about elevated body temperatures

When you’re feeling under the weather, you might reach for the thermometer to see if you have a fever, a telltale sign that your body is fighting an illness. But sometimes, you might have questions about what the thermometer reveals. What temperature is considered a fever? Should you do anything to treat it or let it run its course? And when should you call a doctor, particularly if you discover your child has a fever? 

Consider this your primer on fevers and read on to learn the answers to these questions and more.

What exactly is a fever? 

A fever is not an illness but rather a symptom. It’s a sign that your body is fighting an illness or infection. There’s also evidence that this slight uptick in body temperature works to help control or destroy the invading pathogens that are making you sick while preserving your body’s own cells. A fever might also help stimulate your immune cells to work more effectively. 

Most of the time, you or your child don’t need to be seen immediately by a doctor if you’re experiencing a fever. However, if your child is younger than 3 months old and has a temperature of 100.4°F or higher, you should call your child's health care provider immediately. Fevers in this age group can be a sign of serious infection such as meningitis. Children under 3 months old cannot be given over-the-counter medication to relieve fever. 

Most health care providers define a fever as a temperature of 100.4°F or higher. There are some nuances to note, however. 

• If you take your child’s temperature through their ear, temporally (on the forehead) or rectally, a fever is considered 100.4°F or higher. However, if you take a temperature by placing the thermometer under their arm, nestled in the armpit, a fever is a temperature of 99°F or higher. If you take an oral temperature, a fever is considered 100° F or higher.

• You should not take a child’s temperature using an ear thermometer before they turn 6 months old. 

• When taking a forehead temperature, you should use a digital thermometer. Strips are not considered accurate. 

You may experience a fever with the following common illnesses:

• Flu

• Common cold

• COVID-19

• Urinary tract infection

• Ear infection

• Strep throat

• Food poisoning

• Chicken pox

• Shingles

Additionally, if you get a vaccine such as the flu or COVID-19 shot, you might experience a fever as a side effect to the vaccination.

When should a fever be treated with over-the-counter pain medication?

Fevers can be uncomfortable — you might feel hot and sweaty, achy or so cold that you’re shivering. If you’re having a hard time coping with the symptoms of fever, an over-the-counter pain reliever such as acetaminophen or ibuprofen can help bring your fever down to a comfortable level so you can focus on resting. 

For children older than 3 months, you can treat a fever with acetaminophen if they are experiencing discomfort, usually with a fever of 102°F or higher. Children older than 6 months can be given ibuprofen. Do not give children aspirin due to the risk of Reye syndrome, a rare but serious brain disease. You should also not switch between acetaminophen and ibuprofen while dosing your child. It’s unnecessary and increases the risk of giving them too much pain medication.

Know that treating a fever — whether it’s yours or your child’s — will not help the body get rid of the infection any faster; it simply will relieve discomfort associated with fever. 

Other ways to help you or your child feel more comfortable with a fever include:

• Dressing lightly. Excess clothing will trap body heat and cause your temperature to rise.

• Drinking plenty of fluids.

• Taking a lukewarm, not cold, bath. Never leave your child unattended in the bathtub.

When should I be concerned about a fever? 

If your child is younger than 3 months old and has a temperature of 100.4°F or higher, contact their pediatrician immediately. 

For older children and adults, if a fever is higher than 104°F, lasts longer than 72 hours or comes and goes for more than a week, you should call your doctor or your child’s pediatrician. If you or your child are experiencing other symptoms in addition to the fever that worry you, such as labored breathing or pain that doesn’t go away after taking pain reliever, contact a provider. 

Children with fevers may experience discomfort as their temperature rises. In addition to a body temperature greater than 100.4°F, you might notice:

• Your child may not be as active or talkative as usual.

• They may seem fussier, experience a loss of appetite and become thirstier.

• Your child may feel warm or hot. Remember that even if your child feels like they are "burning up," they might only have a low-grade fever.

What is a febrile seizure? 

Children between the ages of 6 months and 5 years can develop seizures from fever called febrile seizures. If your child does have a febrile seizure, there is a chance that the seizure may occur again, but, usually, children outgrow the febrile seizures. A febrile seizure does not mean your child has epilepsy. There is no evidence that treating the fever will reduce the risk of having a febrile seizure.

If you don't already have a primary care provider, BJC makes it easy to search and schedule an appointment online. You will benefit from virtual care, seamless referrals and direct messaging with your provider. Find a primary care provider near you who is taking new patients.  

If your child has a fever, our Washington University Children’s After Hours locations provide convenient medical care for your child’s illnesses and injuries as a faster alternative to the emergency department when your pediatrician’s office is closed. In most cases, children can be seen here or in a BJC Medical Group Convenient Care, rather than in the emergency department.