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A fever is a temporary increase in your body temperature, often caused by an illness. It is a sign that your body's immune system is actively fighting off an infection.
Evidence checked against current NICE fever guidance for children under 5 and current NHS fever guidance for adults. The recommendations are consistent on assessing the whole clinical picture, using age-specific thresholds and avoiding over-reassurance from temperature alone.
Public health education article
Special Note for Immunocompromised Individuals — Anyone undergoing chemotherapy, taking long-term steroids, or using medications that weaken the immune system should contact a doctor immediately at the first sign of any fever.
HIGH CONFIDENCE
ACT — Know the warning signs and seek urgent care when they apply.
Recognise serious illness early, maintain hydration and comfort, reduce infection transmission, and use age-specific fever guidance.
Status: PENDING
Health Intelligence supports health literacy and editorial transparency. It does not diagnose conditions or replace individual professional assessment.
A fever is a temporary increase in your body temperature, often caused by an illness. It is a sign that your body's immune system is actively fighting off an infection.
Recognise serious illness early, maintain hydration and comfort, reduce infection transmission, and use age-specific fever guidance.
A temperature of 39.4°C (103°F) or higher in an adult
A fever is a temporary increase in your body temperature, often caused by an illness. It is a sign that your body's immune system is actively fighting off an infection.
Normal body temperature usually sits between 36.1°C and 37.2°C (97°F–99°F) and naturally changes throughout the day. A single high reading doesn't tell the whole story; how you or your child feels overall is often more important than the number on the thermometer.
Anyone undergoing chemotherapy, taking long-term steroids, or using medications that weaken the immune system should contact a doctor immediately at the first sign of any fever.
| Age Group | Fever Threshold | What to Do |
|---|---|---|
| Newborns (Under 28 days) | 38°C / 100.4°F or higher | Seek medical care immediately. Do not wait. |
| Infants (1 to 3 months) | 38°C / 100.4°F or higher | Contact a doctor or seek medical care the same day. |
| Children (3 to 36 months) | 38°C / 100.4°F or higher | Monitor closely. Contact a doctor if they are lethargic, distressed, or the fever lasts past 48 hours. |
| Older Children & Teens | 38°C / 100.4°F or higher | Focus on comfort and fluids. See a doctor if symptoms worsen or last more than 3 days. |
| Adults | 38.3°C / 101°F or higher | Rest and stay hydrated. Seek a medical review if the fever hits 39.4°C (103°F) or lasts past 3 days. |
| Older Adults (65+) | 37.8°C / 100°F or higher | Lower baseline temperatures apply. Older adults can have serious infections without a high fever. Early review is best. |
Medical guidelines advise against using fever-reducing medications simply to lower a thermometer reading if the person is otherwise resting comfortably.
BarnMD Health Intelligence uses authoritative and clinically relevant sources appropriate to the topic. References are shown so readers can inspect the evidence directly.
BarnMD Health Library provides general health education for public health literacy. Reading this article does not establish a doctor-patient relationship and this information is not a diagnosis, prescription, or personalised treatment plan. If you have symptoms or concerns about your health, seek appropriate advice from a qualified healthcare professional. If you develop emergency warning signs, seek urgent or emergency medical attention.
A fever is evidence that your immune system is working exactly as it should. Supporting your body with rest and hydration is often the best medicine.
Prioritize how you feel over what the thermometer reads.