High fever in children is most often caused by the body fighting an infection, usually viral. When your child’s immune system detects a virus or bacteria, it triggers a chain reaction that raises body temperature above its normal set point. A rectal temperature of 38°C (100.4°F) or higher is considered a fever, and temperatures above 39.5°C (103.1°F) are generally classified as high fever.
Most fevers in children resolve on their own within a few days and, while alarming, are actually a sign that the immune system is working. Understanding what’s behind the fever helps you gauge whether to ride it out or seek medical attention.
How Fever Works in a Child’s Body
Fever isn’t caused directly by a germ. It’s produced by your child’s own immune system. When the body detects an invader like a virus, bacteria, or toxin, immune cells release signaling molecules called cytokines into the bloodstream. These cytokines travel to a temperature-control center deep in the brain, which functions like a thermostat. There, they trigger the production of a chemical messenger called prostaglandin E2, which effectively turns the thermostat up.
Once the set point rises, the body behaves as though it’s too cold. That’s why a feverish child may shiver, curl up under blankets, or have cold hands and feet even while their core temperature climbs. Blood vessels near the skin constrict to conserve heat, and muscles may shiver to generate it. The fever itself helps fight infection: many viruses and bacteria replicate less efficiently at higher temperatures, and certain immune responses work better in a warmer environment.
Viral Infections: The Most Common Cause
The vast majority of high fevers in children are caused by viral infections. Well over 100 viruses can infect the respiratory tract alone, including rhinoviruses, influenza, parainfluenza, adenoviruses, and respiratory syncytial virus (RSV). Fever from respiratory viruses typically lasts 3 to 5 days.
Influenza stands out for producing particularly high fevers. Temperatures above 39.5°C (103.1°F) occur in more than 50% of children with influenza A. The fever usually comes on suddenly, alongside body aches, fatigue, and cough.
Roseola is another classic cause of high fever in young children and accounts for up to 20% of all febrile illnesses in kids under age 3. It’s distinctive: the fever starts abruptly, often reaching 40 to 41°C (104 to 105.8°F), and persists for 3 to 4 days in about 75% of cases. After the fever breaks, a characteristic pink rash appears on the trunk and spreads outward. Because the fever spikes so suddenly, roseola is a common trigger for febrile seizures.
Other viral culprits include stomach viruses (causing vomiting and diarrhea alongside fever), hand-foot-and-mouth disease, and common childhood illnesses like chickenpox. In most of these cases, the fever resolves as the body clears the virus, and no antibiotic is needed.
Bacterial Infections That Cause High Fever
Bacterial infections are less common than viral ones but tend to be more concerning because they sometimes require antibiotic treatment. The most frequent bacterial causes of fever in children include urinary tract infections, strep throat, ear infections, and pneumonia.
Urinary tract infections are an especially important cause of unexplained fever in young children. A child with a UTI may have no obvious symptoms other than a high fever, particularly if they’re too young to describe pain with urination. The usual culprit is E. coli or other gut bacteria that travel to the urinary tract.
In children over 3 months, the most common serious bacterial infections involve Streptococcus pneumoniae (in unvaccinated children) and Neisseria meningitidis. Strep throat, caused by group A Streptococcus, typically produces fever alongside a very sore throat, swollen tonsils, and sometimes a sandpaper-textured rash. Bacterial pneumonia can cause high fever with rapid breathing, coughing, and low oxygen levels.
Non-Infectious Causes
Not every fever signals an infection. Vaccines commonly cause low-grade fever within a day or two of the shot, though some children develop higher temperatures, especially after certain immunizations. This is a normal immune response and typically resolves within 48 hours.
Heat-related illness can raise a child’s temperature dangerously high, particularly in hot weather or if a child is left in a warm car. Unlike infection-driven fever, heat illness happens because the body absorbs more heat than it can release, and it requires immediate cooling rather than fever-reducing medication.
Rarer causes include autoimmune conditions, certain cancers, and Kawasaki disease, a condition that causes persistent fever lasting five or more days along with red eyes, rash, swollen hands and feet, and cracked lips. These account for a small fraction of childhood fevers but are worth knowing about when a fever doesn’t follow the usual pattern.
Febrile Seizures
Febrile seizures are one of the most frightening things a parent can witness, but they’re relatively common and almost always harmless. They occur in about 2 to 5% of children between the ages of 6 months and 5 years, usually during a rapid spike in temperature rather than at the fever’s peak.
Simple febrile seizures, the most common type, last a few seconds up to 15 minutes and happen only once in a 24-hour period. They affect the whole body. Complex febrile seizures either last longer than 15 minutes, occur more than once within 24 hours, or affect only one part of the body. Simple febrile seizures do not cause brain damage or increase the risk of epilepsy. If your child has one, lay them on their side, don’t put anything in their mouth, and time the episode.
Measuring Your Child’s Temperature
The number on the thermometer depends heavily on where you measure. Normal rectal temperature ranges from 36.6 to 38°C (97.9 to 100.4°F), while an armpit reading runs about two degrees Fahrenheit lower. That difference matters when you’re trying to decide if a temperature is truly elevated.
For children from birth to age 2, a rectal thermometer gives the most accurate reading. From ages 2 to 5, rectal, ear, or armpit measurements all work, though rectal remains the most reliable. Children older than 5 can use an oral thermometer. Armpit readings are fine for screening but can underestimate a true fever.
Managing Fever at Home
Acetaminophen and ibuprofen are the two standard fever reducers for children. Acetaminophen can be given every 4 to 6 hours, with no more than 5 doses in 24 hours. Ibuprofen can be given every 6 to 8 hours, with no more than 4 doses in 24 hours. Both are dosed by weight, not age, so always check the dosing guide on the package against your child’s current weight. Ibuprofen should not be given to infants under 6 months.
Beyond medication, keep your child lightly dressed and offer plenty of fluids. Dehydration is the real danger with prolonged fever, not the temperature number itself. Lukewarm baths can provide some comfort, but avoid cold water or rubbing alcohol, both of which can cause shivering and actually raise core temperature.
The goal of treating a fever isn’t to bring the number back to normal. It’s to help your child feel comfortable enough to rest and drink.
Signs That Need Immediate Attention
Any infant younger than 2 months with a rectal temperature of 100.4°F (38°C) or higher needs emergency evaluation, regardless of how well they appear. At this age, the immune system is immature enough that even a minor-seeming fever can indicate a serious infection.
For older children, the warning signs that warrant a call to your child’s doctor or a trip to the emergency room include:
- Extreme sleepiness or irritability that goes beyond typical sick-day fussiness
- Trouble breathing, including rapid breathing, flaring nostrils, or ribs visibly pulling in with each breath
- A rash accompanying the fever, especially one that doesn’t fade when you press on it
- Stiff neck or severe headache
- Refusing to drink or significantly decreased urination
- Fever lasting longer than three days
- A seizure
- Localized pain, redness, or swelling in one area, such as a very swollen joint or severe throat pain
The height of the fever alone doesn’t always predict how serious the illness is. A child with a 104°F fever who is still drinking, making eye contact, and responding to you is generally less concerning than a child with a 101°F fever who is limp, unresponsive, or refuses all fluids. How your child looks and acts matters more than the number on the thermometer.