For adults, a fever of 103°F (39.4°C) or higher warrants a call to your doctor, and 104°F (40°C) or higher is the widely cited threshold for seeking immediate medical attention. For infants under 3 months old, the bar is much lower: any temperature at or above 100.4°F (38°C) requires prompt medical evaluation. But temperature alone doesn’t tell the whole story. Certain symptoms alongside a fever can turn even a moderate reading into an emergency.
Temperature Thresholds by Age
The number on the thermometer that should prompt action depends heavily on who has the fever. Adults can typically tolerate higher temperatures before the situation becomes dangerous, while babies and older adults need attention sooner.
For infants 0 to 3 months old, a rectal temperature of 100.4°F (38°C) or higher calls for immediate medical evaluation, regardless of how the baby appears. Young infants can’t fight infections the way older children can, and a fever at this age may be the only visible sign of a serious bacterial infection. Don’t give fever-reducing medication and wait to see what happens. Call your pediatrician or go to the emergency room.
For babies 3 to 24 months old, a rectal temperature above 102°F (38.9°C) that lasts longer than one day, or any fever paired with unusual irritability, lethargy, or refusal to eat, should be evaluated by a doctor.
For children over 2, a fever lasting longer than three days deserves a call to their doctor, even if no other symptoms are present.
For adults, contact your doctor at 103°F (39.4°C) or above, and seek immediate help at 104°F (40°C). Most fevers below this range, while uncomfortable, are your immune system doing its job and will resolve on their own.
Why Older Adults Need a Lower Threshold
If you’re over 65 or caring for someone who is, the standard fever cutoffs can be misleading. Roughly 20 to 30 percent of older adults with an active infection show up to the emergency room without a typical fever, because aging blunts the body’s temperature response. Their baseline body temperature also tends to run lower than younger adults.
The current clinical consensus defines a fever in older adults as an ear temperature of 99°F (37.2°C) or higher, or any reading that’s at least 2.3°F (1.3°C) above their personal baseline. This means a reading of 99.5°F in someone who normally runs 97.2°F could signal a serious infection that a standard thermometer cutoff would miss entirely. If an older person seems confused, unusually tired, or “off” even without a high reading, that combination deserves medical attention.
When Fever Is an Emergency at Any Temperature
Sometimes the fever itself isn’t dangerously high, but the symptoms accompanying it are. Call emergency services or go to the ER if a fever comes with any of the following:
- Stiff neck with headache and sensitivity to light: this combination is a hallmark of meningitis
- Seizure or loss of consciousness
- Confusion or difficulty staying awake
- Trouble breathing or rapid breathing
- A rash that doesn’t fade when you press a glass against it: this can indicate sepsis
- Severe pain anywhere in the body
- Pale, mottled, or blotchy skin
- Cold hands and feet despite a high temperature
In babies, watch for a high-pitched cry that sounds different from normal, a bulging soft spot on the head, floppiness or unresponsiveness, and refusal to feed. These are all red flags that warrant emergency care regardless of the exact temperature reading.
The 100.4°F Rule for Cancer Patients
If you’re undergoing chemotherapy or are immunocompromised for any reason, the threshold drops significantly. The CDC classifies a fever of 100.4°F (38°C) or higher during chemotherapy as a medical emergency. Chemotherapy suppresses your body’s ability to fight infection, so what would be a harmless low-grade fever in a healthy person can signal a life-threatening bacterial infection in someone with a compromised immune system.
Take your temperature any time you feel warm, flushed, chilled, or generally unwell during treatment. Don’t wait to see if the fever climbs higher before calling your oncologist.
When a Fever Becomes Dangerous to Your Organs
A body temperature above 106.7°F (41.5°C) is classified as hyperpyrexia, and it’s a medical emergency on its own, separate from whatever is causing it. At this extreme, the heat itself starts damaging your organs. Your brain, heart, lungs, kidneys, and liver all struggle to function when internal temperature climbs this high.
Sustained hyperpyrexia can cause brain swelling, permanent brain damage, coma, and death. This level of fever is rare with common infections. It’s more often seen with heatstroke, certain drug reactions, or severe central nervous system infections. If a thermometer reads anywhere near this range, call emergency services immediately.
How to Get an Accurate Reading
The method you use to take a temperature matters more than most people realize, especially when you’re trying to decide whether to seek care. Rectal and oral thermometers give the most reliable readings. Armpit (axillary) temperatures consistently underestimate true body temperature. Research shows that an armpit reading needs to hit only about 99.5°F (37.5°C) to reliably indicate that an oral reading would show a true fever of 100.4°F (38°C).
For infants, rectal temperature is the gold standard and the measurement that pediatric guidelines are based on. For older children and adults, oral readings are practical and accurate. Forehead and ear thermometers are convenient but can vary depending on technique, so if you get a borderline reading with one of these, confirm it with an oral thermometer before deciding your next step. If you’re using an armpit thermometer and the reading seems lower than expected but the person looks or feels sick, trust the symptoms over the number.