Most fevers in children are caused by viral infections, the common cold, flu, and stomach bugs being the most frequent culprits. A fever is defined as a rectal, ear, or forehead temperature of 100.4°F (38°C) or higher, an oral temperature of 100°F or higher, or an armpit temperature of 99°F or higher. While a fever can be alarming, it’s actually your child’s immune system mounting a defense against whatever is making them sick.
How Fever Works in the Body
Your child’s brain has a built-in thermostat, and fever happens when that thermostat gets deliberately turned up. When the immune system detects an invader like a virus or bacteria, immune cells release chemical signals called pyrogens. These signals travel to the temperature-control center in the brain and trigger the production of specific compounds that raise the body’s set point. Think of it like someone bumping your home thermostat from 72°F to 101°F. The body then works to reach that new target through shivering, constricting blood vessels, and generating heat.
This isn’t a malfunction. The higher temperature makes the body a hostile environment for viruses and bacteria, essentially cooking them out. White blood cells also become more active and respond faster at elevated temperatures. So while fever feels miserable, it’s a sign the immune system is doing its job.
Viral Infections: The Most Common Cause
The vast majority of childhood fevers come from viral illnesses. These include upper respiratory infections (colds), influenza, croup, hand-foot-and-mouth disease, roseola, and stomach viruses. Most viral fevers run their course in three to five days without any specific treatment beyond comfort care. The fever itself isn’t dangerous in these cases, though the child may be irritable, tired, and less interested in eating.
COVID-19 can also cause fever in children, though most kids experience relatively mild symptoms. Respiratory syncytial virus (RSV) is another common source of fever, particularly in infants and toddlers, and can sometimes progress to more serious lower respiratory infections like bronchiolitis.
Bacterial Infections
Bacterial infections are less common than viral ones but tend to be more serious. Urinary tract infections are the most common serious bacterial infection in children younger than three, and they’re a frequent explanation when a child has a fever with no obvious source like a runny nose or cough. Ear infections, strep throat, pneumonia, and skin infections are other bacterial causes that often need antibiotic treatment.
Widespread vaccination against pneumococcal bacteria and Haemophilus influenzae type B has dramatically reduced the rates of bloodstream infections and meningitis in young children. These once-feared complications of childhood fever are now far less common, which is one reason pediatricians approach fever differently today than they did decades ago.
Vaccines and Fever
Post-vaccination fever is a normal immune response and one of the most common non-infectious causes of fever in babies. It occurs most frequently in infants around 3 months old (about 14% of vaccinations), 4 months old (about 12%), and between 12 and 23 months (about 10%). When multiple vaccines are given at the same visit, the chance of fever goes up. One study found that infants who received pneumococcal, Hib, and combination diphtheria-pertussis-tetanus-polio vaccines together had a 17% rate of post-vaccination fever, compared to just 1.3% when none of those vaccines were given.
These fevers are typically low-grade and short-lived. They reflect the immune system learning to recognize the vaccine components, not an infection.
The Teething Myth
Many parents attribute fever to teething, but teething does not cause true fever. It may nudge your baby’s temperature slightly above their baseline, but not high enough to reach 100.4°F. If your baby has a temperature at or above that threshold, something else is going on. The timing is coincidental: babies begin teething around six months, which is also when maternal antibodies start to fade and infections become more frequent.
Inflammatory and Autoimmune Conditions
Less commonly, persistent fever signals an inflammatory condition rather than a straightforward infection. Kawasaki disease is one of the most important to recognize. It causes fever lasting five or more days along with some combination of red eyes, cracked lips, rash, swollen hands or feet, and swollen lymph nodes in the neck. Children with Kawasaki disease are often profoundly irritable, beyond what you’d expect from a typical illness. Early treatment is critical because the condition can damage the heart’s blood vessels.
Multisystem inflammatory syndrome in children (MIS-C) is a related condition that emerged during the COVID-19 pandemic. It causes fever for at least 24 hours, and about 80% of affected children develop significant gastrointestinal symptoms like vomiting, diarrhea, and abdominal pain. Roughly one in five also show neurological symptoms such as severe headache, confusion, or unusual lethargy. MIS-C tends to present with more stomach and brain involvement than Kawasaki disease, and children can appear quite ill, sometimes showing signs of shock.
Overheating and Environmental Causes
Not every elevated temperature reading is a true fever. Overdressing a baby, spending too long in a hot car or direct sun, or vigorous physical activity on a hot day can all push body temperature up. This is hyperthermia, not fever. The difference matters: in true fever, the brain intentionally raises the set point. In hyperthermia, the body simply can’t cool itself fast enough.
Infants are especially vulnerable because they can’t remove blankets or clothing on their own. If your child feels hot after being bundled up or in a warm room, remove a layer, let them cool down for 15 to 20 minutes, and recheck. If the temperature normalizes, it wasn’t a fever.
Managing Your Child’s Fever
Fever reducers like acetaminophen and ibuprofen work by blocking the chemical signals that raise the brain’s temperature set point. Acetaminophen can be given every 4 to 6 hours (no more than 5 doses in 24 hours), while ibuprofen can be given every 6 to 8 hours (no more than 4 doses in 24 hours). Always dose by your child’s weight, not their age. Ibuprofen should not be given to babies under 6 months old.
You don’t have to treat every fever. If your child is comfortable, drinking fluids, and playing, the fever may not need medication at all. The goal of treatment is comfort, not hitting a specific number on the thermometer.
When Fever Needs Urgent Attention
Age is the single most important factor in determining how seriously to take a fever. Any fever in a baby younger than 3 months requires an immediate call to your pediatrician, regardless of how the baby looks. At that age, the immune system is too immature to reliably fight serious infections, and babies can deteriorate quickly without obvious warning signs.
For babies 3 to 6 months old, call if the temperature reaches 100.4°F or if the baby seems unusually sleepy, fussy, or unwell even with a lower temperature. Between 6 and 24 months, a fever above 100.4°F that lasts more than a day warrants a call. For older children, fever lasting more than three days is the general threshold for checking in with a healthcare provider. At any age, a fever paired with a stiff neck, difficulty breathing, a rash that doesn’t fade when you press on it, persistent vomiting, or unusual confusion needs prompt evaluation.