Most fevers in babies are caused by viral infections, the same common bugs that cause colds, flu, and stomach illness. A fever is defined as a rectal temperature of 100.4°F (38°C) or higher. While fevers can be alarming, they’re actually a sign that your baby’s immune system is responding to something, whether that’s a routine virus, a bacterial infection, or even a recent vaccination.
Viral Infections Are the Most Common Cause
The vast majority of fevers in babies come from everyday viral illnesses. Upper respiratory infections (common colds) top the list, followed by the flu, stomach viruses, and bronchiolitis caused by RSV. These infections typically run their course within a few days to a week without needing antibiotics.
A few viral illnesses stand out in infancy. Roseola is a classic one: your baby spikes a high fever for several days, and just when the fever breaks, a pink rash appears across the chest and spreads outward. Chickenpox, hand-foot-and-mouth disease, and croup are other common culprits. Ear infections, while sometimes bacterial, often start with a viral infection that leads to fluid buildup behind the eardrum.
One reassuring detail from the research: when doctors can confirm a virus is causing a baby’s fever, the risk of a serious bacterial infection happening at the same time drops significantly. The exception is urinary tract infections, which still occur in about 3.3% of infants with bronchiolitis.
Bacterial Infections: Less Common but More Serious
Bacterial infections cause a smaller share of infant fevers, but they demand closer attention because they can escalate quickly without treatment. In babies younger than 3 months with a fever, roughly 6 to 10% turn out to have a serious bacterial infection. Urinary tract infections are the most frequent among these.
Thanks to widespread vaccination, bloodstream infections in children have dropped dramatically. Only about 1 in 200 febrile children now test positive for bacteria in their blood, down from much higher rates in earlier decades. The bacteria most commonly responsible are types that cause pneumonia, ear infections, and urinary tract infections.
The youngest babies face the highest risk. In newborns under 21 days old, the chance of an invasive bacterial infection like bloodstream infection or meningitis is estimated at 3 to 5%. That risk drops substantially after the first few weeks of life. This is why age matters so much when evaluating a baby’s fever.
Fever After Vaccinations
Vaccines are a normal and expected cause of fever in babies. Most post-vaccination fevers appear within a week of the shot, though the exact timing varies depending on the type of vaccine. Vaccines that use weakened live viruses (like the MMR vaccine given later in infancy) tend to cause fever a bit later than vaccines made from inactivated components, which typically trigger a fever within a day or two.
These fevers are generally mild and short-lived. They reflect the immune system doing exactly what it’s supposed to do: learning to recognize and fight a pathogen. If your baby recently had a vaccination and develops a low-grade fever without other concerning symptoms, the vaccine is the most likely explanation.
Teething Does Not Cause True Fevers
This is one of the most persistent myths in parenting. Researchers have studied it carefully and found that while teething can cause a slight uptick in body temperature, the increase stays below the 100.4°F threshold that defines a true fever. If your baby has a temperature at or above 100.4°F, something other than teething is responsible, even if new teeth happen to be coming in at the same time.
The timing creates a misleading pattern. Babies start teething around 4 to 6 months, which is also when the protective antibodies they got from their mother begin to fade. More frequent infections at this age get wrongly blamed on teething.
Overheating and Environmental Causes
Sometimes what looks like a fever is actually overheating. Babies, especially newborns, regulate their body temperature less efficiently than older children. Overbundling, warm rooms, car seats with heavy blankets, or direct sunlight can all push a baby’s core temperature up. If your baby feels hot, try removing a layer of clothing or moving to a cooler room and recheck the temperature after 15 to 20 minutes. A true fever will persist; overheating will resolve once the external heat source is removed.
Why Age Changes Everything
A fever that would be routine in a toddler can be a medical emergency in a newborn. The younger the baby, the higher the risk that a fever signals a serious infection, and the harder it is to tell a dangerous infection from a harmless virus based on appearance alone.
For babies under 21 days old, any fever of 100.4°F or higher triggers a full evaluation, including blood work, urine testing, and a spinal fluid check, along with hospitalization while results come back. This sounds aggressive, but the risk of meningitis and bloodstream infections at this age justifies the caution. Between 22 and 28 days, the approach is similar but slightly less intensive depending on initial test results. From 29 to 60 days, doctors use blood markers to help decide which babies can safely be monitored at home and which need admission.
After 3 months, the calculus shifts. Babies this age with fevers are far less likely to have a serious bacterial infection, and doctors can rely more on how the baby looks and behaves to guide decisions.
Signs That a Fever Needs Urgent Attention
The fever number itself matters less than what’s happening alongside it. Several specific signs raise the concern for serious infection:
- Poor arousability. A baby who is unusually difficult to wake, limp, or unresponsive to stimulation.
- Petechial rash. Tiny flat purple or red dots on the skin that don’t fade when you press on them. This can signal a bloodstream infection.
- Labored breathing. Flaring nostrils, grunting, or skin pulling in between the ribs with each breath.
- Slow capillary refill. If you press on your baby’s fingertip and the color takes more than three seconds to return, circulation may be compromised.
- Behavioral change. Persistent inconsolable crying, refusing to eat, or appearing unusually still and disengaged.
No single sign reliably rules out a serious infection on its own. What matters is the overall picture: a baby who is alert, feeding, making eye contact, and producing wet diapers is far more reassuring than one who is lethargic and disinterested in eating, regardless of what the thermometer says.
How to Get an Accurate Reading
The method you use to take your baby’s temperature matters. A rectal thermometer gives the most reliable reading in infants and is the standard doctors use. The threshold for fever is 100.4°F (38°C) rectally. Armpit (axillary) readings are convenient but less accurate, with a fever threshold of 99°F (37.2°C). If an armpit reading seems borderline or doesn’t match how your baby looks, follow up with a rectal measurement to be sure.