Fever in children is almost always caused by the immune system fighting off an infection, most commonly a virus. When your child’s body detects an invader, it deliberately raises its internal temperature to create a less hospitable environment for germs and to boost immune cell activity. A rectal, ear, or forehead temperature of 100.4°F (38.0°C) or higher counts as a fever, while an oral reading of 100°F (37.8°C) or higher qualifies.
How a Fever Actually Works
Fever isn’t a malfunction. It’s a coordinated response controlled by the brain’s thermostat, a cluster of temperature-sensing neurons deep in the hypothalamus. When immune cells in the blood and liver detect a pathogen, they release signaling molecules called cytokines. These signals travel to the brain through two routes: one carried by the bloodstream and another transmitted through nerve fibers connected to the organs.
Once the signals arrive, they trigger the production of a chemical messenger (a prostaglandin called PGE2) at the surface of the brain’s thermostat. This messenger essentially turns up the set point, the way you’d crank a dial on a thermostat from 98.6°F to, say, 102°F. The brain then sends out commands to raise body temperature to match: blood vessels near the skin constrict to trap heat, sweating slows down, metabolism speeds up, and your child starts shivering. That shivering isn’t a sign of cold. It’s the body generating heat to reach the new, higher target temperature.
Viral and Bacterial Infections
The vast majority of childhood fevers come from common viral infections. Colds, flu, stomach bugs, croup, and respiratory syncytial virus (RSV) are frequent culprits, especially in children under five who are encountering these germs for the first time. Most of these fevers resolve on their own within a few days as the immune system clears the virus. COVID-19 can also cause fever in children, though most kids experience relatively mild symptoms.
Bacterial infections are less common but tend to be more serious. Urinary tract infections are the most common serious bacterial infection in children under three, and they can cause fever without any other obvious symptoms, which is why doctors often check a urine sample when a young child has an unexplained fever. Ear infections, skin infections, bone infections, and pneumonia are other bacterial sources. During flu season, children under three who test positive for influenza actually have low rates of a simultaneous serious bacterial infection, which can help guide how aggressively doctors need to investigate.
Vaccines and Post-Shot Fevers
Vaccines work by stimulating the immune system, so a mild fever afterward is a normal sign that the body is responding. The timing and likelihood vary by vaccine. For most shots, fever starts within 12 to 24 hours and lasts one to two days. The DTaP vaccine triggers fever in about 25% of children. The pneumococcal vaccine causes mild fever (under 102°F) in about 15%. The flu shot produces a low-grade fever in roughly 20% of kids.
A few vaccines follow a different timeline. The MMR vaccine can cause fever 6 to 12 days after the shot, not the next day, which catches some parents off guard. It affects about 10% of children and lasts two to three days. The chickenpox vaccine similarly produces a mild fever 14 to 28 days later in about 10% of kids. The rotavirus vaccine, notably, causes no fever at all.
Teething: Not the Fever You Think
Teething is one of the most commonly blamed causes of fever, but the evidence tells a more nuanced story. The largest clinical study on teething symptoms did find a statistically significant association between teething and a slight rise in temperature. However, healthcare professionals caution against casually attributing fever to teething because no specific pattern of symptoms can reliably distinguish teething from the early stages of a serious infection. A baby who is irritable and warm may be teething, or may be getting sick.
If your child’s temperature reaches 100.4°F or higher, it’s worth considering illness as the cause rather than assuming teeth are to blame, especially in infants under 12 months who are both actively teething and highly vulnerable to infections.
Non-Infectious Causes
Not all fevers come from germs. Heat exhaustion can raise a child’s core temperature after prolonged exposure to hot environments or intense physical activity. Unlike infection-driven fevers, this type of overheating isn’t orchestrated by the immune system and requires cooling the child down rather than waiting it out.
Some children experience recurring fevers with no infection behind them at all. A group of conditions called systemic autoinflammatory diseases cause the innate immune system to activate inappropriately, producing inflammation and fever on a repeating cycle. Most are genetic. The most common is familial Mediterranean fever, which causes painful inflammation in the abdomen, chest, and joints. Another, called PFAPA, typically starts before age four and produces predictable episodes of fever, mouth sores, sore throat, and swollen glands. Episodes often resolve on their own after age ten. If your child gets fevers on a regular, almost clockwork schedule without signs of infection, these conditions are worth discussing with a pediatrician.
When a Fever Signals Something Serious
Most fevers in children are harmless and resolve within a few days. But certain signs alongside a fever indicate the need for immediate evaluation. Any fever in a baby under 28 days old is treated as a medical emergency, full stop. Babies from birth to three months with any fever need to be seen by a doctor promptly, regardless of how well they seem.
In older infants and children, the child’s behavior matters more than the number on the thermometer. A child who looks seriously ill after their fever has been brought down with medication is more concerning than one who spikes a high number but perks up once it drops. Specific red flags include lethargy or listlessness, difficulty breathing, a rash of small purple or red dots that don’t fade when you press on them, and inconsolable crying. A child who is overly compliant and limp is actually more worrying than one who is cranky and fighting you.
If a fever climbs higher than it was, your child looks progressively sicker, or new symptoms appear, those are all reasons to seek care again even if you’ve already had the child evaluated.
Managing Fever at Home
Fever itself rarely needs aggressive treatment. The goal of giving fever-reducing medication is comfort, not hitting a specific number. Acetaminophen is the most widely used option and is dosed by your child’s weight (not age, when possible). The standard pediatric liquid form contains 160 mg per 5 mL. For children under 12, it can be given every four hours as needed, up to five doses in 24 hours. Children under two should not receive acetaminophen without a doctor’s guidance.
Use an oral syringe to measure liquid medication, never a kitchen spoon. Avoid combination products (those containing multiple active ingredients) for children under six. And if your child is sleeping comfortably with a fever, there’s no need to wake them for a dose. The fever is doing its job.
Measuring Temperature Accurately
The method you use to take your child’s temperature affects the reading. Rectal thermometers are the gold standard for accuracy in young children. Ear thermometers are unreliable before six months of age. An armpit reading of 99°F (37.2°C) or higher suggests fever, but it’s the least precise method and can underestimate the true core temperature by a degree or more. For a quick check, forehead thermometers work reasonably well, but if the reading seems borderline or your child seems sicker than the number suggests, a rectal measurement gives the clearest answer.