What is a fever?
Normal body temperature varies between individuals and throughout the day — typically ranging from 97°F to 99°F (36.1°C to 37.2°C), with the lowest point in the early morning and the highest in the late afternoon. A fever is defined as a core body temperature at or above 100.4°F (38.0°C) measured orally.
Fever is not a disease — it is a physiological response. When the immune system detects pathogens or damage signals, it releases pyrogens (interleukins, prostaglandins) that act on the hypothalamus — the brain's thermostat — to raise the body's temperature set-point. This higher temperature inhibits the replication of many bacteria and viruses, enhances immune cell activity, and accelerates inflammatory responses.
This means that in most cases, a modest fever is actually helpful. Aggressively suppressing every fever with antipyretics may slightly prolong viral illnesses. The priority is always comfort and identifying any serious underlying cause — not normalising the thermometer reading at all costs.
Oral (mouth): Most accurate for adults. Wait 15 minutes after eating or drinking. Hold under tongue with mouth closed for 1 minute. Rectal: Most accurate for infants and young children; 0.5°F higher than oral. Axillary (armpit): Least accurate; 0.5–1°F lower than oral — not recommended for clinical decisions. Tympanic (ear): Convenient but affected by technique and ear canal anatomy. Temporal artery (forehead): Quick screening but can read low if technique is poor. Avoid: Forehead strip thermometers — highly inaccurate.
Temperature guide for adults
| Temperature (oral) | Classification | Recommended action |
|---|---|---|
| Below 100.4°F / 38.0°C | Normal / no fever | No action needed for temperature alone; address other symptoms |
| 100.4–101.9°F / 38.0–38.8°C | Low-grade fever | Rest, hydration, monitor. Antipyretics if uncomfortable. See doctor if persists >3 days or worsens. |
| 102–103°F / 38.9–39.4°C | Moderate fever | Rest, hydration, antipyretics for comfort. See doctor within 24 hours if no obvious cause or if high-risk group. |
| 103–104°F / 39.5–40.0°C | High fever | Antipyretics; contact doctor same day. ER if not responding to medication or if red flags present. |
| Above 104°F / 40.0°C | Very high fever | Seek immediate medical care. ER or 911 if not rapidly responding to antipyretics. |
| Above 106°F / 41.1°C | Hyperpyrexia — emergency | Call 911 immediately — risk of brain damage and multi-organ failure |
When to seek emergency care
Stiff neck + fever + headache (possible meningitis) · Petechial or purpuric rash (non-blanching spots — possible meningococcal disease) · Confusion, altered consciousness, or seizure · Temperature above 104°F (40°C) not responding to antipyretics · Severe difficulty breathing · Chest pain with fever · Severe abdominal pain · Fever in an immunocompromised person (chemotherapy, HIV, organ transplant, steroids) · Fever in anyone returning from malaria-endemic region · Fever in an infant under 3 months
Common causes of fever
| Category | Common causes | Distinguishing features |
|---|---|---|
| Viral infections | Influenza, COVID-19, common cold, EBV (glandular fever), RSV, norovirus | Most common cause; usually self-limiting; antibiotics ineffective; may have cough, sore throat, myalgia, fatigue |
| Bacterial infections | Urinary tract infection (UTI), pneumonia, strep throat, cellulitis, sinusitis, otitis media | Often higher fever; localising symptoms (dysuria, cough+sputum, ear pain); may require antibiotics |
| Severe bacterial infection / sepsis | Sepsis, meningitis, endocarditis, pyelonephritis, intra-abdominal abscess | Very high fever or hypothermia; rapid heart rate, low BP, confusion — emergency |
| Inflammatory / autoimmune | Rheumatoid arthritis flare, lupus, inflammatory bowel disease, vasculitis, adult Still's disease | Relapsing pattern; associated joint pain, rash, or organ-specific symptoms; elevated CRP/ESR |
| Drug fever | Antibiotics (especially beta-lactams), anticonvulsants, allopurinol, heparin, many others | Occurs 1–2 weeks after starting medication; relative bradycardia; rash in 20–30%; resolves on drug withdrawal |
| Cancer | Lymphoma, leukaemia, renal cell carcinoma, hepatocellular carcinoma | Persistent or relapsing fever; night sweats; unexplained weight loss; enlarged lymph nodes |
| Post-vaccination | Influenza, COVID-19, MMR, hepatitis B vaccines | Low-grade fever within 12–48 hours; self-limiting; normal and expected immune response |
| Heat-related illness | Heat exhaustion, heat stroke | Not true fever — thermoregulatory failure; occurs in hot environments; heat stroke is emergency |
| Traveller's fever | Malaria, typhoid, dengue, chikungunya, rickettsial disease | Always ask about recent travel; malaria must be excluded in any returned traveller with fever |
Fever in children
Fever management in children requires extra caution because risk thresholds differ significantly by age. The younger the child, the greater the concern — because immature immune systems mount less robust localised responses and serious infections can deteriorate rapidly with few warning signs.
| Age group | Fever threshold | Recommended action |
|---|---|---|
| Under 3 months | 100.4°F / 38.0°C or above (rectal) | Emergency — ER immediately regardless of how well baby appears. Risk of serious bacterial infection (SBI) is highest in this age group. |
| 3–6 months | 100.4°F / 38.0°C | Contact doctor urgently. ER if appears unwell, very irritable, or temperature above 102°F. |
| 6–24 months | 102°F / 38.9°C or higher, or lower with red flags | See doctor same day if fever lasts more than 24 hours, temperature above 104°F, or child appears very unwell. |
| 2–5 years | 102–104°F / 38.9–40°C | Monitor at home if child appears well and is drinking fluids; see doctor if lasting more than 3 days or worsening. |
| Over 5 years | As per adult guidance | Home management if well; seek care for high fever, red flags, or duration beyond 3–5 days. |
Red flags in children with fever — seek immediate care
- Non-blanching rash (petechiae or purpura) — press a glass against it; if it doesn't fade, call 999/911 immediately
- High-pitched, unusual, or continuous cry
- Inconsolable — cannot be soothed
- Pale, mottled, ashen, or blue skin
- Bulging fontanelle in infants
- Stiff neck
- Seizure (febrile convulsion) — especially if first seizure, duration over 5 minutes, or focal seizure
- Not drinking fluids at all; dry mouth, sunken eyes, no tears (dehydration)
- Difficult or laboured breathing; grunting; nasal flaring
Febrile seizures occur in 2–4% of children aged 6 months to 5 years. They are caused by a rapid rise in temperature rather than the absolute temperature level. Simple febrile seizures (generalised, lasting under 5 minutes, in a neurologically normal child) are frightening but generally benign — they do not cause brain damage and do not significantly increase the risk of epilepsy. Place the child on their side (recovery position), do not put anything in their mouth, time the seizure, and call 911 if it lasts more than 5 minutes. All first febrile seizures should be evaluated medically.
Fever in vulnerable adults
Certain groups of adults require lower thresholds for seeking medical attention with fever:
- Immunocompromised individuals: Those receiving chemotherapy, biological therapies, high-dose corticosteroids, organ transplant recipients, or people with HIV/AIDS are at high risk of rapidly progressing bacterial infections that can present with surprisingly modest fever. Neutropenic fever (fever in a patient with low neutrophil count) is a haematological emergency — call 911 or go to ER immediately.
- Adults over 65: Older adults may have a blunted febrile response — serious infections may present with lower temperatures, confusion, or falls rather than high fever. A temperature of 99°F or above in a frail older adult warrants evaluation.
- Pregnant women: Fever in pregnancy — particularly in the first trimester — is associated with neural tube defects and miscarriage. High fever (above 102°F / 38.9°C) in any trimester warrants prompt medical contact. Paracetamol/acetaminophen is safe; ibuprofen should be avoided after 20 weeks of pregnancy.
- People with diabetes: Infections can rapidly destabilise blood sugar control and cause DKA. Any significant fever in a diabetic patient warrants checking blood glucose frequently and contacting their diabetes team.
- People with asplenia: Surgical or functional loss of the spleen (sickle cell disease, splenectomy) dramatically increases risk of overwhelming post-splenectomy infection (OPSI) — particularly from encapsulated bacteria. These patients should be vaccinated against pneumococcus, meningococcus, and Haemophilus influenzae, and any fever requires urgent same-day medical evaluation.
Home care and treatment
For most healthy adults with moderate fever from a clearly identifiable viral cause (common cold, flu, COVID-19), home management is entirely appropriate:
| Measure | Evidence | Practical guidance |
|---|---|---|
| Rest | Essential — fever increases metabolic demand | Rest allows immune resources to focus on fighting infection; avoid strenuous activity during fever |
| Hydration | Critical — fever significantly increases fluid and electrolyte loss | Drink 2–3 litres per day; water, diluted juice, broth, or oral rehydration solutions (ORS); avoid alcohol |
| Paracetamol / acetaminophen | Effective antipyretic and analgesic — first-line | Adults: 500–1000 mg every 4–6 hours; maximum 4g/day; safe in pregnancy; do not exceed dose — hepatotoxic in overdose |
| Ibuprofen | Equally effective antipyretic — also anti-inflammatory | Adults: 200–400 mg every 6–8 hours with food; avoid in kidney disease, peptic ulcer, third trimester of pregnancy, asthma if NSAID-sensitive |
| Alternating paracetamol and ibuprofen | Moderate evidence for children; sometimes used in adults | Can provide more consistent temperature control and pain relief — but complexity increases risk of dosing errors; use clear written schedule |
| Tepid sponging | Limited evidence — provides temporary comfort only | Use lukewarm (not cold) water; avoid shivering (which raises temperature); may help with comfort when medication alone insufficient |
| Light clothing and cool environment | Practical comfort measure | Avoid heavy blankets or excessive clothing; cool room (18–20°C); fan if comfortable |
Aspirin (acetylsalicylic acid) in children and teenagers with viral illness is associated with Reye's syndrome — a rare but serious condition causing liver and brain damage. Use only paracetamol/acetaminophen or ibuprofen in children. Aspirin is safe in adults.
Fever of unknown origin (FUO)
Fever of unknown origin (FUO) is defined as fever above 38.3°C (101°F) persisting for more than 3 weeks with no identified cause after an initial investigation. It represents a diagnostic challenge requiring systematic workup. The classic triad of causes is:
- Infections (30–40%): Tuberculosis, endocarditis, intra-abdominal abscess, osteomyelitis, HIV, and endemic mycoses are the most common. Tuberculosis is the most common cause of FUO worldwide.
- Neoplasms (20–30%): Lymphoma (especially Hodgkin's and non-Hodgkin's), leukaemia, and solid tumours (renal cell carcinoma, hepatocellular carcinoma) frequently cause fever.
- Non-infectious inflammatory diseases (10–20%): Adult Still's disease (characterised by quotidian fever, salmon-coloured rash, and arthritis), systemic lupus erythematosus, vasculitis (temporal arteritis, polyarteritis nodosa), and inflammatory bowel disease.
- Other causes (<10%): Drug fever, pulmonary embolism, factitious fever, and a proportion (10–15%) that remains undiagnosed despite extensive workup.
Investigation of FUO is guided by clinical history and examination findings and typically includes: extended blood cultures, comprehensive blood tests (FBC, inflammatory markers, LFTs, LDH, ferritin, ANCA, ANA, complement), CT chest/abdomen/pelvis, and PET-CT scan — which has transformed the diagnostic workup by identifying occult malignancy, infection foci, and inflammatory conditions with high sensitivity.
Frequently asked questions
In adults, a fever is defined as an oral temperature of 100.4°F (38.0°C) or above. A temperature of 100.4–103°F (38.0–39.4°C) is considered low-grade to moderate fever. Above 103°F (39.4°C) warrants same-day medical attention. Above 104°F (40°C) is a high fever requiring prompt emergency evaluation, and 106°F (41.1°C) or above is hyperpyrexia — a life-threatening emergency. Note that older adults may have serious infections at temperatures below the standard fever threshold; any unexplained new temperature above 99°F in a frail older adult warrants evaluation.
Not necessarily — fever is a beneficial immune response that helps the body fight infection. Most fevers do not need to be suppressed unless they are causing significant discomfort, are very high (above 103°F / 39.4°C), or occur in vulnerable groups (infants, pregnant women, immunocompromised individuals). Antipyretics relieve discomfort but do not treat the underlying cause and may slightly prolong some infections. The priority is always identifying and treating the underlying cause, maintaining hydration, and monitoring for warning signs — not simply normalising the temperature reading.
Fever without an obvious infectious source can be caused by autoimmune and inflammatory conditions (lupus, adult Still's disease, vasculitis, inflammatory bowel disease), certain cancers (lymphoma, leukaemia, renal cell carcinoma), medications (drug fever — occurring 1–2 weeks after starting a new drug and resolving when it is stopped), and occasionally non-infectious conditions like deep vein thrombosis or pulmonary embolism. Fever persisting more than 3 weeks without an identified cause is termed fever of unknown origin (FUO) and requires systematic specialist investigation.
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- Fever in children: NICE traffic light system. NICE Clinical Knowledge Summaries. 2023.
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- Sullivan JE, Farrar HC. Fever and antipyretic use in children. Pediatrics. 2011;127(3):580–587.