What it is
Fever is an elevation in body temperature above the normal range (typically >38°C/100.4°F), representing an active physiological immune response.
An elevation in body temperature above 38°C, usually representing the immune system's response to infection or inflammation.

At a glance
What it is
Fever is an elevation in body temperature above the normal range (typically >38°C/100.4°F), representing an active physiological immune response.
Commonly experienced as
Evidence context
Research-supportedSafety
See staying safeHistory & Origin
Fever is defined as a core body temperature above 38°C (100.4°F), produced by the hypothalamic thermostat resetting in response to pyrogens — chemical signals released by immune cells during infection, inflammation, or tissue injury. It is one of the most common presenting symptoms across all age groups and clinical settings. Causes range from viral upper respiratory tract infections (the most common) to bacterial infections (urinary tract infection, pneumonia, meningitis), autoimmune disease, malignancy, medication reactions, and heat-related illness. Associated symptoms (localising features such as cough, dysuria, rash, or neck stiffness) are critical for identifying the underlying cause. High or prolonged fever in vulnerable populations (infants, elderly, immunocompromised) requires urgent assessment.
The Evidence
What the evidence says about fever — its causes, when to act, and how different health approaches understand it.
A well-understood immune response with clear clinical thresholds
Fever is one of the most studied symptoms in medicine, with strong evidence guiding when to monitor, when to treat the underlying cause, and when to seek urgent care. Most fevers in healthy adults resolve without intervention, but specific features demand prompt professional assessment.
Fever with neck stiffness, a non-blanching rash, or sensitivity to light may indicate meningococcal disease — call emergency services immediately. Any fever in an infant under 3 months requires urgent assessment. Fever with confusion in someone who is immunocompromised may signal sepsis. Fever above 40°C or lasting more than two weeks without a clear cause also requires prompt professional evaluation.
Paracetamol and ibuprofen are well-evidenced for reducing fever and associated discomfort, though they do not shorten illness duration in uncomplicated viral infections. Antibiotics are appropriate only for confirmed or strongly suspected bacterial causes — using them for viral fever carries no benefit and contributes to antimicrobial resistance. Watchful waiting is supported for uncomplicated viral illness in otherwise healthy adults.
Aspirin carries a risk of Reye's syndrome — a rare but serious condition — when used in children under 16 with fever. Paracetamol or ibuprofen are the appropriate options in this age group. Antibiotic use for viral fever should be avoided; it offers no benefit and increases resistance risk. Anyone who is immunocompromised, pregnant, elderly, or has a chronic condition should seek professional guidance early rather than self-managing.
The body raises temperature in response to infection, inflammation, or tissue injury as part of normal immune function. Associated symptoms — such as cough, urinary discomfort, rash, or joint pain — help identify the underlying cause. Common causes include viral respiratory infections, urinary tract infections, and post-vaccination responses. Less common causes include autoimmune flares, malignancy, and medication reactions.
Naturopathy and traditional European herbal medicine have historically supported mild fever rather than suppressing it, using diaphoretic herbs such as elderflower and yarrow. Ayurveda and Traditional Chinese Medicine each have distinct frameworks for understanding and responding to fever patterns. These approaches are adjunctive and do not replace professional assessment for significant, prolonged, or concerning fever.
A general practitioner or urgent care provider is the appropriate first contact for fever that is high, prolonged, or accompanied by concerning symptoms. Complementary or traditional practitioners may offer supportive care alongside conventional management for mild, uncomplicated cases — but should not be the sole point of contact when red flag features are present. Always inform all practitioners of any medications or supplements being used.
Safety first
General, informational guidance — not diagnostic. A qualified practitioner can advise on your own situation.
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