Fever in children and babies: what to do and when to go to the ER

A fever is a temperature of 100.4 °F (38.0 °C) or higher. The fever itself is not dangerous: it is a sign the body is fighting an infection. If the fever is an adult’s, read fever in adults.

Use a digital thermometer, never a mercury one. For babies, a rectal reading is the most accurate. Forehead (temporal artery) thermometers work at any age, ear thermometers from 6 months, and oral readings only from age 4. Forehead strips and armpit readings are less reliable. Write down each reading, the time and any medicine you gave.

Dr. Diana Nieves Castro, a general practice physician licensed in Puerto Rico with several years of emergency-department experience, sees children of all ages, babies included, in English or Spanish. We evaluate, diagnose and treat; if needed, we give you a lab order for a lab near you, such as a urine test or blood work.

The doctor looks at your child on camera, asks how the fever started, looks for the cause (a cold or cough, ear pain, hand, foot and mouth disease…) and gives you the right dose for their weight. If appropriate, the doctor prescribes and we send your prescription to your pharmacy. More on our online doctor visit for kids.

If your child is under 21, a parent with patria potestad (parental authority) or a legal guardian stays on camera for the whole visit and shows photo ID. Read how to prepare your child for a video visit.

Video doctor visits 24/7 — book online and choose your time right away.

It is almost always a viral infection. The causes table on this page lists the usual ones: colds, flu, ear pain, hand, foot and mouth disease, diarrhea and vomiting, urinary tract infections and, in Puerto Rico, dengue. Other symptoms help the doctor tell which it is. According to the CDC, a high fever with cough, runny nose, red eyes and then a rash that starts on the face can be fever with a rash (measles): call ahead before going to a health facility.

Dengue hits children and teens hard on the island: in 2024, 10- to 19-year-olds were 29.3% of cases, according to the CDC. Warning signs usually appear 24 to 48 hours after the fever goes away, so keep watching your child once it breaks. If dengue is possible, use acetaminophen and do not give ibuprofen. Read our dengue warning signs guide.

If your child has a fever and no warning signs:

• Offer fluids often and let them rest.
• Acetaminophen or ibuprofen can make your child more comfortable; you do not need to treat the number if they are resting comfortably.
• Dose by weight, not just age. Use the syringe or cup that comes with the medicine, never a kitchen spoon.
• Do not give acetaminophen under age 2 without a doctor's guidance, and do not give ibuprofen to babies 6 months or younger.
• Never give aspirin to a child unless their doctor tells you to.
• Do not overdress them.

Even if your child looks well, get them checked if:

• Your child is under 2 and the fever lasts more than 24 hours without a clear cause. A urine test may be needed: the doctor gives you a lab order for a lab near you and goes over the result with you.
• Your child is 2 or older and the fever lasts more than 3 days (72 hours).
• The temperature keeps rising above 104 °F (40 °C).
• For their safety, if the fever lasts 5 days or more, at any age, have your child examined in person.

For their safety, if your baby is under 3 months and has a temperature of 100.4 °F (38.0 °C) or higher, go to an emergency room or call 911, even if the baby seems fine and even in the middle of the night. If your baby is 3 to 6 months old and reaches 102.2 °F (39 °C) or higher, get them seen in person soon.

At any age, call 911 or go to the ER if your child has a fever plus any warning sign on this page. In babies, also watch for a bulging soft spot, fewer than 6 wet diapers a day, or green vomit. In children 1 to 5, a high-pitched noise when breathing at rest (stridor), or heavy drooling and not being able to swallow. In older children and teens, severe pain in the lower right belly, testicular pain, or an asthma attack that does not improve with their inhaler.