Prevention & care

Fever: temperature measurements, infection, age and warning signs

7 sources · Latest source check 2026-10-13 UTC

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Fever is a temporary rise in body temperature associated with illness. It is a sign, rather than a diagnosis of a particular infection. Temperature normally varies with the individual, time of day, activity and measurement site. A number therefore needs its context, including how it was measured and what else is happening.[1]

Young infants and serious associated symptoms need prompt help

A baby younger than three months with 38°C or higher needs prompt medical advice. Child emergency signs include breathing difficulty, being difficult to wake, a stiff neck or a nonfading rash. Use the local emergency number, such as 911 in the US. Responsiveness and feeding also matter.[3]

Report the method as well as the reading

Mouth, armpit, rectal, ear and forehead measurements do not necessarily give identical values. Device technique, recent exercise, a bath and hot or cold drinks can influence a reading. Use the device instructions and record the site and time. MedlinePlus notes that the exact relationship between methods is uncertain, even though average differences are described. Adding a fixed correction to every home reading can suggest more precision than the measurement supports.[4]

Fever does not distinguish viral from bacterial illness

Infections are common causes, but fever can also accompany inflammatory disease, some medicine reactions and other conditions. A high fever can occur with a mild viral illness, while a serious infection may cause little fever or an abnormally low temperature. Clinicians consider duration, associated symptoms, exposures, medical history and examination when selecting tests. The temperature alone does not tell whether antibiotics are useful or identify the organ involved.[1]

Recognize a concerning whole-body illness

Sepsis is a life-threatening response to infection requiring immediate evaluation. CDC lists possible features including confusion, severe discomfort, clammy skin, fever or feeling very cold, a rapid heart rate or weak pulse, and shortness of breath. Not every person has every listed symptom, and fever by itself does not diagnose sepsis. If an infection is worsening and these features are present, seek emergency care and communicate the concern rather than waiting for a thermometer to reach a particular value.[5]

Overheating is a separate emergency mechanism

Heat stroke is dangerous overheating with signs such as confusion, slurred speech, collapse or seizures, sometimes with hot dry skin and sometimes with sweating. It needs emergency help and cooling while help arrives, following emergency-service instructions. It differs from treating discomfort during an ordinary infection-related fever. Do not wait for a precise thermometer result when someone exposed to heat becomes confused or collapses. A high temperature is a shared observation; its cause determines the appropriate response.[6]

Evidence & uncertainty

NHS adult advice commonly uses 38°C or higher as a high-temperature threshold. Other clinical resources use measurement-specific definitions or consider changes from a person’s usual temperature. These descriptions are not a universal severity scale. Someone can feel hot or shivery below 38°C, and an illness assessment considers more than whether one cutoff was crossed.[2]

Selected-source overview · Independent review pending

Published questions

Is 37°C everyone’s exact normal temperature?

Selected-source overview · Independent review pending. Read the evidence context.

No. Normal temperature varies across people, time of day and measurement method. Tell a clinician the reading, device site and circumstances rather than reporting only that it was above average.[4]

Evidence behind this answer

Temperature measurement
agency hosted clinical explanation · 2026
Selected relevant passages on the original source page inspected · Preparation; normal variation; uncertain relationship between sites