Babies get fevers because their immune system is doing exactly what it’s designed to do: raising the body’s internal temperature to fight off an infection. When a baby’s body detects a virus or bacteria, immune cells release chemical signals that travel to the brain’s temperature control center and essentially turn up the thermostat. A fever in an infant is defined as a rectal temperature of 100.4°F (38°C) or higher.
This process is the same one adults experience, but babies get fevers more frequently because their immune systems are encountering common germs for the very first time. Almost every childhood infection, from a simple cold to an ear infection, can set it off.
How a Fever Actually Works
When bacteria or a virus enters your baby’s body, immune cells respond by releasing substances called pyrogens. These trigger the production of a chemical messenger, prostaglandin E2, which signals the hypothalamus (the brain’s thermostat) to raise the body’s temperature set point. The body then acts as if its normal temperature is too low: your baby may shiver, feel cold to the touch, or want to be held close. All of this is the body working to reach that new, higher set point.
The higher temperature serves a real purpose. It makes the body a hostile environment for viruses and bacteria, which reproduce less efficiently in the heat. White blood cells also become more active at elevated temperatures, responding to infection faster. In short, fever isn’t the illness itself. It’s a weapon against the illness.
The Most Common Causes
The vast majority of infant fevers are caused by viral infections. Colds, flu, RSV, and croup are among the most frequent culprits. These infections are typically mild and resolve on their own, but they almost always bring a fever along for the ride. Babies catch these viruses easily because they haven’t built up immunity to the dozens of common germs circulating in any household or daycare.
Bacterial infections are less common but tend to be more serious. Ear infections, which often develop after a cold, are one of the most familiar examples. Around 10% of young infants with a fever turn out to have a urinary tract infection, which can be easy to miss because the only symptom may be the fever itself. Throat infections, sinus infections, pneumonia, and intestinal infections can also be responsible.
In rare cases, a fever can signal something more dangerous, such as a bloodstream infection (sepsis) or meningitis, an infection of the membranes surrounding the brain and spinal cord. These are uncommon but are the reason fevers in very young infants are taken seriously by doctors.
Vaccines and Post-Shot Fevers
Routine vaccinations are another common reason babies develop fevers, and these fevers are a normal sign that the immune system is responding to the vaccine. Most post-vaccination fevers appear within a day or two and last about a day. The DTaP vaccine, one of the first shots babies receive, causes a fever in roughly 8 to 20 out of every 100 recipients. The timing varies by vaccine type: some cause fevers within hours, while the chickenpox vaccine can trigger a low fever up to two to four weeks later as the weakened virus in the vaccine stimulates a slow immune response.
Does Teething Cause Fevers?
This is one of the most persistent beliefs in parenting, and the answer is more nuanced than a simple yes or no. A large meta-analysis found no overall association between tooth eruption and fever. However, when researchers looked specifically at rectal temperature readings, there was a measurable link: babies were nearly three times more likely to have a slightly elevated temperature around the time a tooth broke through. The key word is “slightly.” Teething may raise a baby’s temperature just enough to feel warm, but it doesn’t typically push the thermometer to 100.4°F or beyond. If your baby has a genuine fever, it’s worth looking for another cause rather than assuming it’s teething.
Fever vs. Overheating
Not every high temperature reading means your baby has a fever. Hyperthermia, or simple overheating, looks similar on a thermometer but has a completely different cause. It happens when a baby’s body absorbs more heat than it can release, often because of too many layers of clothing, heavy swaddling, or a warm room. Unlike a true fever, which the brain deliberately creates to fight infection, overheating is uncontrolled and has nothing to do with the immune system.
If you suspect your baby is overdressed or the room is too warm, remove a layer and recheck the temperature in 15 to 20 minutes. A true fever won’t come down just from removing a blanket.
How to Get an Accurate Reading
The method you use to take your baby’s temperature matters. A rectal thermometer is the most accurate option for infants and is the standard pediatricians rely on. Temporal artery (forehead) and ear thermometers use the same fever threshold of 100.4°F. An armpit reading runs lower, so a fever by that method is 99°F or above. For babies under three months, a rectal reading is strongly recommended because even small inaccuracies from other methods can affect medical decisions.
When a Fever Needs Immediate Attention
A baby’s age is the single most important factor in determining how urgently a fever needs to be evaluated. For any infant under 3 months, a rectal temperature of 100.4°F or higher warrants a call to the pediatrician right away, even if the baby seems fine otherwise. This is because young infants are more vulnerable to serious bacterial infections and their symptoms can be subtle.
The American Academy of Pediatrics breaks this down further. Babies 8 to 21 days old with a fever are evaluated the most aggressively, typically with blood and urine tests, a spinal fluid sample, and antibiotics started before results come back. Babies 22 to 28 days old undergo similar testing, though the approach may be adjusted based on initial results. By 29 to 60 days, doctors have a bit more flexibility and may skip the spinal fluid test if other markers look reassuring. These protocols exist because the younger the baby, the harder it is to distinguish a mild illness from a dangerous one based on appearance alone.
For babies older than 3 months, the fever number matters less than how the baby is acting. A baby who is alert, feeding, and making wet diapers is generally in a different category than one who is listless, difficult to wake, or refusing to eat.
Febrile Seizures
About 3 to 4 out of every 100 children will experience a febrile seizure, a convulsion triggered by a rapid rise in body temperature. These are most common between 12 and 18 months of age, though they can occur anytime between 6 months and 5 years. Watching your child have a seizure is terrifying, but febrile seizures are almost always harmless and don’t cause brain damage or epilepsy. They tend to last less than a minute or two, and most children who have one never have another. The seizure is triggered by how quickly the temperature rises, not by how high it gets, which is why they often occur at the very start of an illness before parents even realize their child has a fever.