What is a fever?
Normal body temperature varies between individuals and throughout the day — typically 97°F to 99°F (36.1°C to 37.2°C), lowest in the early morning and 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 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. Aggressively suppressing every fever may slightly prolong viral illnesses.
Oral: Most accurate for adults. Wait 15 min after eating/drinking. Under tongue, mouth closed, 1 minute. Rectal: Most accurate for infants — 0.5°F higher than oral. Axillary (armpit): Least accurate; 0.5–1°F lower than oral — not for clinical decisions. Tympanic (ear): Quick but technique-dependent. Temporal artery (forehead): Convenient but can read low. Avoid: Forehead strip thermometers — highly inaccurate.
Temperature guide for adults
| Temperature (oral) | Classification | Action |
|---|---|---|
| <100.4°F / 38.0°C | Normal | No action 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 >3 days or worsening. |
| 102–103°F / 38.9–39.4°C | Moderate fever | Rest, hydration, antipyretics. See doctor within 24 hours if no obvious cause or high-risk group. |
| 103–104°F / 39.5–40.0°C | High fever | Antipyretics; contact doctor same day. ER if not responding or red flags present. |
| Above 104°F / 40.0°C | Very high fever | Seek immediate medical care. |
| Above 106°F / 41.1°C | Hyperpyrexia — emergency | Call 911 immediately — risk of brain damage and organ failure |
When to call 911 or go to ER immediately
Stiff neck + fever + headache (possible meningitis) · Petechial or purpuric rash (non-blanching spots — possible meningococcal disease) · Confusion or altered consciousness · Seizure · Temperature above 104°F (40°C) not responding to antipyretics · Severe difficulty breathing · Fever in immunocompromised person (chemotherapy, HIV, transplant) · Fever in infant under 3 months (any temperature)
Common causes of fever
| Category | Common examples | Key features |
|---|---|---|
| Viral infections | Influenza, COVID-19, EBV, RSV, norovirus | Most common; self-limiting; antibiotics ineffective; cough, sore throat, myalgia |
| Bacterial infections | UTI, pneumonia, strep throat, cellulitis, sinusitis | Often higher fever; localising symptoms; may require antibiotics |
| Sepsis | Severe bacterial infection spreading to bloodstream | Very high or very low temperature; rapid heart rate; low BP; confusion — emergency |
| Inflammatory / autoimmune | RA flare, lupus, IBD, vasculitis, adult Still's disease | Relapsing pattern; joint pain, rash, or organ symptoms; elevated CRP/ESR |
| Drug fever | Antibiotics, anticonvulsants, allopurinol, heparin | Occurs 1–2 weeks after starting drug; resolves on withdrawal |
| Cancer | Lymphoma, leukaemia, renal cell carcinoma | Persistent or relapsing; night sweats; weight loss; lymphadenopathy |
| Post-vaccination | Influenza, COVID-19, MMR vaccines | Low-grade, within 12–48 hours; self-limiting; normal immune response |
| Traveller's fever | Malaria, typhoid, dengue | Always ask about recent travel; malaria must be excluded in any returned traveller with fever |
Fever in children
| Age group | Threshold | Action |
|---|---|---|
| Under 3 months | 100.4°F / 38.0°C (rectal) | ER immediately regardless of how well baby appears |
| 3–6 months | 100.4°F / 38.0°C | Contact doctor urgently. ER if unwell or above 102°F. |
| 6–24 months | 102°F or lower with red flags | See doctor same day if fever >24 hours or temperature above 104°F. |
| 2–5 years | 102–104°F | Monitor at home if well and drinking; see doctor if lasting >3 days or worsening. |
| Over 5 years | As per adult guidance | Home management if well; seek care for high fever, red flags, or duration >3–5 days. |
Red flags in children with fever — seek immediate care
- Non-blanching rash — press a glass against it; if it doesn't fade, call 911
- Inconsolable crying or high-pitched unusual cry
- Pale, mottled, ashen, or blue skin
- Bulging fontanelle in infants
- Stiff neck or photophobia
- Seizure — especially first seizure, duration over 5 minutes, or focal seizure
- Not drinking at all; sunken eyes; no tears when crying (dehydration)
- Laboured breathing, grunting, or nasal flaring
Aspirin in children and teenagers with viral illness is associated with Reye's syndrome — rare but serious liver and brain damage. Use only paracetamol/acetaminophen or ibuprofen in children. Aspirin is safe in adults.
Fever in vulnerable adults
- Immunocompromised: Chemotherapy, biologics, transplant, HIV — neutropenic fever is a haematological emergency. Any fever in these patients warrants same-day medical evaluation at minimum.
- Over 65: Older adults may have blunted fever response — serious infection may present with confusion or falls rather than high temperature. Any unexplained temperature above 99°F in a frail older adult warrants evaluation.
- Pregnant women: Fever above 102°F in any trimester warrants prompt medical contact. Paracetamol/acetaminophen is safe; ibuprofen should be avoided after 20 weeks.
- Asplenic patients: Surgical or functional asplenia (sickle cell, splenectomy) — risk of overwhelming post-splenectomy infection. Any fever requires same-day emergency evaluation.
Home care for fever
| Measure | Evidence | Practical guidance |
|---|---|---|
| Rest | Essential | Fever increases metabolic demand — avoid strenuous activity |
| Hydration | Critical | Drink 2–3 litres/day; water, broth, ORS; avoid alcohol |
| Paracetamol/acetaminophen | First-line | Adults: 500–1000 mg every 4–6 hours; max 4g/day; safe in pregnancy; hepatotoxic in overdose |
| Ibuprofen | Equally effective + anti-inflammatory | Adults: 200–400 mg every 6–8 hours with food; avoid in kidney disease, peptic ulcer, third trimester |
| Tepid sponging | Modest comfort benefit | Lukewarm water only — cold water causes shivering which raises temperature |
| Light clothing | Comfort measure | Avoid heavy blankets; cool room 18–20°C; fan if comfortable |
Fever of unknown origin (FUO)
FUO is defined as fever above 38.3°C (101°F) persisting for more than 3 weeks with no identified cause after initial investigation. The three classic categories:
- Infections (30–40%): Tuberculosis (most common worldwide), endocarditis, intra-abdominal abscess, osteomyelitis, HIV, endemic mycoses.
- Neoplasms (20–30%): Lymphoma, leukaemia, solid tumours (renal cell carcinoma, hepatocellular carcinoma) — "B symptoms" (fever, night sweats, weight loss) are characteristic of lymphoma.
- Non-infectious inflammatory (10–20%): Adult Still's disease (quotidian fever + salmon rash + arthritis), lupus, vasculitis, IBD.
Investigation is guided by clinical history and includes extended blood cultures, comprehensive blood tests, CT chest/abdomen/pelvis, and PET-CT — which has transformed FUO workup by identifying occult malignancy and infection foci 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. Above 103°F warrants same-day medical attention. Above 104°F is a high fever requiring prompt emergency evaluation. Above 106°F (41.1°C) is hyperpyrexia — a life-threatening emergency requiring immediate call to 911.
Not necessarily. Fever is a beneficial immune response. Antipyretics relieve discomfort but do not treat the underlying cause and may slightly prolong some infections. The priority is hydration, rest, monitoring for warning signs, and identifying any serious underlying cause — not simply normalising the thermometer reading at all costs. Treat for comfort when fever is causing significant distress, is very high, or occurs in vulnerable groups.
Any fever (100.4°F / 38.0°C or above, measured rectally) in a baby under 3 months old is a medical emergency requiring immediate ER evaluation regardless of how well the baby appears. The risk of serious bacterial infection — meningitis, sepsis, urinary tract infection — is highest in very young infants, and babies of this age can deteriorate very rapidly. Do not wait to see if it resolves.
- NICE. Fever in under 5s: assessment and initial management. NICE guideline NG143. 2021.
- Knockaert DC, et al. Fever of unknown origin in adults: 40 years on. J Intern Med. 2003;253(3):263–275.
- Niven DJ, et al. Accuracy of peripheral thermometers for estimating temperature. Ann Intern Med. 2015;163(10):768–777.
- Sullivan JE, Farrar HC. Fever and antipyretic use in children. Pediatrics. 2011;127(3):580–587.