Fever in babies is most often caused by the immune system fighting off a viral infection, though bacterial infections, vaccinations, and even overdressing can raise a baby’s temperature. A rectal temperature of 100.4°F (38°C) or higher is considered a fever in infants of any age.
Understanding what’s behind your baby’s fever helps you respond calmly and know when the situation needs medical attention.
How a Baby’s Body Produces a Fever
Fever isn’t a disease. It’s a defense mechanism. When your baby’s immune system detects an invader like a virus or bacteria, immune cells release signaling chemicals called pyrogens. These signals travel to the brain’s internal thermostat, a region called the hypothalamus, which controls body temperature the same way a thermostat controls the heat in your house.
Normally, the hypothalamus keeps body temperature hovering around 98.6°F. During an infection, those pyrogen signals reset the thermostat to a higher “target” temperature. Your baby’s body then acts as though its normal temperature is too cold, triggering responses to generate and conserve heat: shivering, reduced blood flow to the skin, and increased metabolic activity. The result is a fever, which creates an environment that makes it harder for many pathogens to thrive and helps immune cells work more efficiently.
Viral Infections: The Most Common Cause
The vast majority of fevers in babies come from common viral illnesses. Colds, flu, stomach bugs, and respiratory viruses are the usual suspects. Babies encounter these viruses frequently, especially once they start daycare or have older siblings bringing germs home.
Respiratory syncytial virus (RSV) is one worth knowing about. It’s extremely common in young children, but it doesn’t always follow the pattern parents expect. Many infants with RSV don’t develop a fever at all. Instead, the early signs are a runny nose, reduced appetite, and a cough that can progress to wheezing or difficulty breathing. In very young babies under 6 months, RSV may show up as irritability, decreased activity, poor feeding, or brief pauses in breathing lasting more than 10 seconds. The illness often starts mild and worsens a few days in, so a baby who seems fine at first can deteriorate quickly.
Other common viral triggers include roseola (which typically causes several days of high fever followed by a rash), hand-foot-and-mouth disease, and various upper respiratory infections. Most viral fevers resolve on their own within three to five days as the immune system clears the infection.
Bacterial Infections to Watch For
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 under three years old, and their frequency is increasing. UTIs in babies don’t look the way they do in adults. There’s no complaint of burning or urgency. Fever may be the only visible symptom, which is why doctors routinely check urine when a young child has an unexplained fever.
Bacterial pneumonia is another possibility, particularly if your baby has a cough along with rapid breathing, visible pulling in of the skin between the ribs, or low oxygen levels. Ear infections, skin infections, and less commonly, bloodstream infections or meningitis can also cause fever in babies. The younger the infant, the higher the concern, because a newborn’s immune system is still immature and less equipped to contain a bacterial infection on its own.
Post-Vaccination Fever
A mild fever after routine vaccinations is one of the most common side effects and is generally harmless. It means your baby’s immune system is responding to the vaccine and building protection. These fevers, along with redness or swelling at the injection site, typically last one to two days and resolve without treatment.
Not every vaccine causes fever equally. Some, like the combination vaccines given at two, four, and six months, are more likely to trigger a temperature spike than others. If your baby develops a fever within 24 to 48 hours of a vaccination and has no other concerning symptoms, the vaccine is the most likely explanation.
Teething Does Not Cause True Fever
This is one of the most persistent myths in parenting. Teething does not cause true fever. It can lead to a slightly elevated temperature, but studies consistently show that teething-related temperature stays below the 100.4°F fever threshold. If your teething baby has a rectal temperature at or above 100.4°F, something else is going on, and it’s worth investigating rather than attributing it to teeth coming in.
The timing is what creates the confusion. Babies begin teething around six months, which is also when the passive immunity they received from their mother during pregnancy starts to fade. They get sick more often, and parents naturally connect the fever to the most visible change: swollen gums.
Overheating and Overbundling
Sometimes a high temperature reading isn’t a fever at all. Newborns and young infants can overheat when dressed in too many layers, wrapped in heavy blankets, placed near heaters or heat vents, or left in a warm car. This raises their core temperature without any infection or immune response involved.
If you suspect overheating, remove a layer or two and recheck your baby’s temperature after 15 to 20 minutes. A true fever will persist. An overheating-related temperature will drop once the extra heat source is removed. This distinction matters, especially in newborns, where a confirmed fever triggers a very different medical response than simple overdressing.
What Counts as a Fever by Age
The threshold for concern depends on your baby’s age. A rectal thermometer is the most accurate method for infants.
- Under 3 months: A rectal temperature of 100.4°F (38°C) or higher is serious and needs immediate evaluation at an emergency department, regardless of how well your baby appears.
- 3 to 6 months: A temperature up to 101°F (38.3°C) warrants attention if your baby seems unusually irritable, sluggish, or uncomfortable. Anything above 101°F at this age should be evaluated.
- 6 to 12 months: A temperature above 101°F (38.3°C) that lasts longer than one day without other symptoms, or any fever paired with concerning behavior, needs a doctor’s assessment.
The reason the under-3-month threshold is so strict: young infants can have serious bacterial infections, including meningitis and bloodstream infections, with fever as the only early sign. Doctors take any confirmed fever in this age group seriously and will typically run blood tests, urine tests, and sometimes a spinal fluid analysis to rule out dangerous infections before they progress.
Managing Your Baby’s Fever
For babies older than 8 weeks, acetaminophen (Tylenol) can help bring down a fever and improve comfort. Ibuprofen (Motrin, Advil) is an option starting at 6 months of age but should not be given to younger infants. Never give acetaminophen to a baby under 8 weeks old without medical guidance, and always dose by weight rather than age for accuracy.
Beyond medication, keep your baby lightly dressed, offer frequent feedings to prevent dehydration, and let them rest. A lukewarm bath can help temporarily but avoid cold water, which causes shivering and can actually raise core temperature. The goal isn’t to eliminate the fever entirely. It’s to keep your baby comfortable while their immune system does its work.
Signs That Need Immediate Attention
Most fevers in babies are caused by routine infections and resolve without incident. But certain red flags signal something more urgent:
- Any fever in a baby under 3 months: Go to the emergency department.
- Drowsiness or difficulty waking: This goes beyond normal sleepiness from illness.
- Difficulty breathing: Rapid breathing, grunting, or visible rib pulling.
- A stiff neck: A possible sign of meningitis.
- A rash that doesn’t fade when pressed: Could indicate a serious infection.
- Vomiting and refusing fluids: Raises the risk of dehydration.
- A bulging soft spot (fontanelle): Suggests increased pressure inside the skull.
- Temperature above 104°F (40°C): Warrants prompt medical evaluation.
- Fever lasting more than two days: Even without other symptoms, a persistent fever deserves investigation.
- A febrile seizure: While usually not dangerous on their own, seizures triggered by fever should always be evaluated.
Trust your instincts as a parent. If your baby has a fever and something feels off, even if you can’t pinpoint exactly what, that’s reason enough to seek medical advice.