Kids get fevers because their immune system is doing exactly what it’s supposed to do: raising the body’s internal temperature to fight off an infection. Fever isn’t an illness itself. It’s a defense mechanism, triggered most often by common viral infections like colds, flu, and stomach bugs. Understanding how this process works can help you tell the difference between a routine fever and one that needs attention.
How a Fever Actually Works
Your child’s brain has a built-in thermostat, a small region that constantly monitors and regulates body temperature. When the immune system detects an invader like a virus or bacterium, immune cells release signaling molecules that travel through the bloodstream to this thermostat. Those signals trigger the production of a compound called prostaglandin E2, which effectively turns up the set point, much like adjusting a thermostat dial from 98.6°F to 101 or 102°F.
Once the set point rises, the body treats its current normal temperature as “too cold.” That’s why your child shivers, burrows under blankets, and feels chilled even though their skin is hot to the touch. The shivering generates heat, blood vessels near the skin constrict to retain it, and the body climbs toward the new target temperature. When the infection starts to resolve, the set point drops back down, and your child sweats to cool off.
Why the Body Bothers With Fever
Fever isn’t just a side effect of being sick. Elevated temperatures actively help the immune system work better. Lab studies show that temperatures in the fever range (around 104°F) can reduce the replication rate of certain viruses by more than 200-fold. Higher temperatures also make some bacteria more vulnerable to destruction by the immune system. At the same time, fever stimulates both the fast-response and longer-term branches of immunity, helping white blood cells move more quickly to the site of infection and mount a stronger attack.
This is why many pediatricians emphasize that a mild fever in an otherwise comfortable child doesn’t always need to be treated. The fever is doing useful work.
The Most Common Causes in Children
The vast majority of childhood fevers come from viral infections. Colds, influenza, respiratory syncytial virus (RSV), stomach viruses, and hand-foot-and-mouth disease are among the most frequent culprits. These fevers typically resolve on their own within a few days as the immune system clears the virus.
Bacterial infections can also cause fevers, though they’re less common. Ear infections, urinary tract infections, strep throat, and pneumonia are the usual suspects. Since the introduction of vaccines for pneumococcal disease and Hib (a type of bacterial meningitis), the rates of serious bacterial infections causing fever in young children have dropped significantly. COVID-19 can cause fever in children too, though most kids experience relatively mild symptoms.
During flu season, children under three who test positive for influenza have low rates of serious bacterial infection happening at the same time, which is reassuring. Similarly, research has found that young infants who test positive for RSV are at reduced risk for a concurrent bacterial infection.
Vaccines
Childhood immunizations are another common and completely normal trigger. Post-vaccination fevers typically start about 9 to 15 hours after the shot, depending on the vaccine, and usually last less than 48 hours. Pneumococcal vaccines tend to cause the shortest fevers (resolving around 12 hours after onset), while influenza and hepatitis A vaccines may produce fevers that last a bit longer. These fevers are a sign the immune system is responding to the vaccine and building protection.
Teething
Parents often blame teething for fevers, and there’s a kernel of truth here, but it’s smaller than most people think. The largest clinical study on teething symptoms did find a statistically significant association with mild temperature elevation, but true fevers above 100.9°F (38.3°C) were uncommon. If your child has a temperature above 101°F and is teething, an infection is a more likely explanation than the tooth itself.
Why Kids Get Fevers More Often Than Adults
If it feels like your child catches every bug that circulates through daycare or school, you’re not imagining it. Children’s immune systems are still learning. Each new virus or bacterium is a first encounter, and the body mounts a full immune response, fever included, to fight it and build lasting immunity. Adults have already encountered many of these pathogens and carry immune memory that can fight them off before a fever ever develops. On average, young children get six to eight viral infections per year, each one potentially bringing a fever along with it.
Managing a Fever at Home
The goal of treating a fever isn’t necessarily to bring the number on the thermometer back to normal. It’s to keep your child comfortable. If your child has a mild fever but is playing, drinking fluids, and acting mostly like themselves, treatment may not be needed at all.
When a fever is making your child miserable, acetaminophen (Tylenol) or ibuprofen (Motrin, Advil) are the standard options. Acetaminophen can be given to children of all ages, while ibuprofen is typically used for children six months and older. Both are dosed by weight, not age, so check the packaging carefully or ask your pharmacist. Liquid children’s acetaminophen comes in a standardized concentration of 160 mg per 5 mL, which the FDA recommended in 2011 to reduce dosing errors. Don’t give more than five doses in 24 hours for children under 12, and never give aspirin to children due to the risk of a rare but serious condition called Reye’s syndrome.
Keep your child hydrated with water, diluted juice, popsicles, or an oral rehydration solution. Dress them in light clothing and avoid bundling them in heavy blankets, especially once the fever starts to break and they begin sweating.
Febrile Seizures
Some children experience seizures triggered by fever, known as febrile seizures. These occur most often between 6 months and 5 years of age, with the highest risk between 12 and 18 months. During a febrile seizure, a child may shake uncontrollably, stiffen, or become briefly unresponsive. These episodes are terrifying to watch but are usually brief and don’t cause lasting harm. If your child has one, lay them on their side on a safe surface, don’t put anything in their mouth, and contact your doctor afterward. Febrile seizures don’t mean your child has epilepsy, and most children who have one never have another.
Fevers That Need Medical Attention
Age matters more than the number on the thermometer, especially for very young infants. Any rectal temperature of 100.4°F (38.0°C) or higher in a baby 3 months old or younger is considered a potential emergency. Infants this young have immature immune systems that can’t reliably fight serious infections, and fever may be the only visible sign of something dangerous. Babies under 28 days old with a fever are typically hospitalized for testing and monitored for at least 36 hours.
For older babies and children, pay closer attention to how your child looks and acts than to the exact temperature reading. Seek immediate medical care if your child has any of the following alongside a fever:
- Extreme sleepiness or unresponsiveness: difficulty waking up or not reacting to your voice and touch
- Difficulty breathing: fast, labored, or shallow breaths, chest pulling inward with each breath, or blue-tinged lips
- Signs of dehydration: dry mouth, no tears when crying, fewer than six wet diapers in 24 hours, or a sunken soft spot on a baby’s head
- A rash that doesn’t fade when pressed, or purple spots on the skin, which could indicate a serious bacterial infection like meningitis
- A stiff neck or resistance to bending the head forward
- A seizure, particularly if it lasts more than five minutes or your child doesn’t return to normal behavior afterward
A child who has a high fever but is alert, drinking fluids, and engaging with you is in a very different situation from a child with a lower fever who is limp, pale, and unresponsive. Behavior is the most reliable signal that something serious may be going on.