Fever in Babies

What a fever is, the signs, and what causes it

A fever is a body temperature of 100.4°F (38°C) or higher, measured rectally for the most accurate reading in babies and young toddlers. It is not a disease — it is a sign that the immune system has turned up the body's internal thermostat, usually to fight an infection. The number itself tells you surprisingly little about how sick your baby is, which is why doctors care far more about how your baby looks and acts than about the exact reading. One rule sits above everything else: in any baby under 3 months old, a fever of 100.4°F (38°C) or higher always needs prompt medical attention — call your doctor or seek urgent care before giving any medicine. Beyond the thermometer, a feverish baby is often fussier than usual, feeds less, sleeps more or sleeps poorly, and may have flushed cheeks, warm skin, sweating, or shivering; a warm forehead alone is not reliable, so confirm with a thermometer. Most fevers come from ordinary viral infections — colds, the flu, roseola, RSV, or stomach bugs — that clear on their own, though bacterial infections such as ear or urinary tract infections can also cause fever and sometimes need treatment. Two myths are worth retiring: routine vaccines can trigger a mild, short-lived fever in the day or two afterward (an expected immune response), and teething may cause warmth and crankiness but does NOT cause a true fever — so never blame a real fever on teeth.

Caring for a fever at home

For babies 3 months and older who are otherwise doing okay, the goal is comfort — not chasing the temperature down to a normal number. Keep fluids going with more frequent breast or formula feeds; offer small sips of water only if your baby is already on solids and your doctor has approved it. Dress your baby in one light layer rather than bundling, keep the room comfortable instead of hot, and skip cold baths, ice packs, and rubbing alcohol, all of which can cause shivering or harm. Fever-reducing medicine (infant acetaminophen, or ibuprofen only for babies 6 months and older) is sometimes appropriate when an older baby is genuinely uncomfortable — but always confirm the right product, the correct weight-based amount, and the timing with your pediatrician or pharmacist first, and never medicate a baby under 3 months before they are seen. Two firm safety lines: never give aspirin to a baby or child, because of its link to the rare but dangerous Reye's syndrome; and never use honey under 1 year of age, because of the risk of infant botulism. Throughout, watch the whole child, not the number — a baby who is alert, drinking, and soothable is generally doing well even with a temperature.

When to call the doctor — the real red flags

Call your doctor right away, day or night, for ANY fever in a baby under 3 months old — the single most important warning on this page. For older babies, call if the fever lasts more than about two to three days, keeps returning after seeming to break, climbs to a high reading such as 104°F (40°C), or — far more telling than any number — comes with a baby who looks or acts more unwell than the fever alone would explain. Also call for signs of dehydration: very few wet diapers, no tears when crying, a dry mouth, or a sunken soft spot. Seek emergency care immediately if your baby is hard to wake or unusually limp, is breathing fast or struggling to breathe, has blue or gray lips or skin, has a stiff neck, has a bulging soft spot, has a seizure, or has a high-pitched or inconsolable cry. A rash that does not fade when you press a clear glass against it is a medical emergency — but do not wait for it, because meningitis can strike before any rash appears or with a rash that still fades, so trust the other warning signs too. Repeated forceful vomiting, or simply a baby who seems very unwell to you, also warrants being seen now. When in doubt, get them checked — no one will fault you for being careful with a baby.

The bottom line

Most fevers in babies are the body's healthy, expected response to a passing infection, and many resolve within a few days with rest, fluids, and gentle comfort. The temperature number matters far less than how your baby looks, feeds, and whether they can be soothed — so watch the whole child, not just the thermometer. Hold one clear line in mind: under 3 months, any fever means call the doctor first and do not medicate before your baby is seen. For older babies, comfort care is usually enough, and you can always check with your pediatrician or pharmacist before giving any medicine. This page is educational and is not a substitute for advice from a provider who knows your baby — when something feels off, reaching out early is exactly the right move.