When your baby spikes a fever, you can't help but worry. A fever—or febrile illness—in infants is something to see a pediatrician about right away because it is the most common sign of childhood illness. 

Since infants with their still-developing immune systems are so vulnerable to infection, most fevers (above 100.4 degrees Fahrenheit) experienced during the first three months of life are taken seriously by pediatricians and emergency medicine doctors alike. 

Illnesses that bring on fevers in infants are usually caused by viral infections, but about 10 percent are caused by potentially more serious bacterial infections, says Paul L. Aronson, MD, a Yale Medicine pediatric emergency specialist. These can include urinary tract infections, blood infections or bacterial meningitis—an infection of the fluid surrounding the brain and spinal cord.

Our experts are actively investigating ways to improve how infants with fevers are treated. Yale New Haven Children's Hospital is joining other hospitals around the country in an effort to determine the best way to treat this age group when they experience a fever. 

If an infant in the first 90 days of life has a temperature of 100.4 degrees Fahrenheit or above (optimally taken rectally for accuracy), a pediatrician should be consulted immediately. Older babies and children should also receive medical attention when needed for fevers, too.

Examination may lead a pediatrician to find, for example, an ear infection (otitis media), pneumonia, tonsillitis or gastroenteritis, among other illnesses. 

Unlike adults or older children, some infants might not have any other symptoms except for the fever. But other symptoms of illness or infection can include:

• fussiness
• a change in feeding behavior 
• sleepiness/difficulty waking up

If the fever is caused by a virus, it might be accompanied by a runny nose, sneezing and/or coughing. These symptoms may also be present with a serious bacterial infection, and a pediatrician can decide if emergency care is warranted. 

Young infants with infections commonly have fever as a symptom. Premature babies, infants with chronic illnesses (such as heart disease), and babies born to mothers who have a bacterial infection (such as group B strep) are at an increased risk of having a fever due to a serious bacterial infection.

Sometimes young children who get fevers will experience febrile convulsions are seizures (fits or convulsions) without another underlying cause. Children who experience febrile seizures should receive immediate medical attention.

The diagnosis is made with a clinical examination and laboratory testing. “We observe how the baby looks overall,” says Dr. Aronson. 

“We get aggressive with our diagnostic tests and treatment if the baby looks ill, but even if the baby looks well, laboratory testing is necessary to evaluate for a bacterial infection.” A urine test and blood test will probably be taken right away.

While bacterial cultures are the definitive tests used to diagnose a serious bacterial infection, it may take a day or two before the results are known. Therefore clinicians rely on the baby’s medical history, appearance and results of more rapidly available urine and blood tests to determine if the baby might have a more serious bacterial infection.

A spinal tap may also need to be performed to evaluate for meningitis if the baby is very young (less than one month of age) or if he or she has other concerning signs or symptoms, or abnormalities in initial blood testing.

“A spinal tap understandably causes parents the most anxiety,” Dr. Aronson says. “It involves drawing fluid through a tiny puncture in the baby’s lower back with a small needle.” Spinal fluid is obtained below the level of the actual spinal cord, so there is a low risk of damaging the spinal cord, he says. “There are risks of introducing an infection or bleeding with the procedure, but these risks are also very low.”

If bacterial meningitis goes undetected and/or untreated, it can cause neurologic damage, hearing loss, kidney failure, seizures, learning disabilities or even death.

Infants one month old or younger would likely be admitted to the hospital to receive antibiotics for 24 to 48 hours. It might take hours or days to get results from a urine, blood or spinal fluid bacterial culture, which is why many infants are treated with antibiotics immediately, even if a diagnosis is not yet conclusive.

Some babies older than one month might be sent home without antibiotics if the baby looks well and has reassuring results of the laboratory testing available in the emergency department. Families with infants discharged from the emergency department should follow up with their pediatrician the next day.

If there is concern about bacterial infection, babies may be admitted to the hospital to be treated with antibiotics. If antibiotics are started early, the outcomes for infants with bacterial infection are good.

If no bacterial infection is found, a baby will continue to be monitored, either in the hospital or at home, to make sure that any viral infections are clearing up on their own.

“The research I’m involved with seeks to improve strategies for caring for young infants with fevers,” Dr. Aronson says. “Because serious infections are rare yet dangerous, the key is to figure out how not to under- or over-treat babies. By identifying infants at both high and low risk for bacterial infection, we hope to better target testing and antibiotics to those infants most vulnerable," he says.