Fever — Causes, Treatment & When to Seek Emergency Care — Causes, Diagnosis & When to See a Doctor | MyMedicPlus

Fever is a temporary, regulated elevation of the body's core temperature above 38°C (100.4°F), driven by the hypothalamic thermostat in response to pyrogens — substances released by immune cells (prostaglandin E2 being the primary mediator) in response to infection, inflammation, or tissue damage. Unlike hyperthermia (unregulated rise from heat exposure or metabolic causes), fever is an active, purposeful immune mechanism: elevated temperature inhibits bacterial and viral replication, enhances neutrophil and macrophage activity, and optimises antibody production. Normal core body temperature is approximately 37°C (98.6°F), though this varies slightly between individuals, by time of day (lowest in early morning, highest in late afternoon), and by measurement site (rectal measurement is most accurate, being 0.3–0.5°C higher than oral, and 0.5–1°C higher than axillary). Most fevers in community settings are caused by self-limiting viral infections and resolve without intervention within 3–5 days. However, fever can also be the first sign of a life-threatening bacterial infection such as meningitis, pneumonia with sepsis, or pyelonephritis progressing to urosepsis. In certain vulnerable populations — infants under 3 months, immunocompromised individuals, patients on immunosuppressive medications, and recently returned travellers from malaria-endemic regions — fever requires mandatory urgent medical evaluation because the range of serious causes is broader and the ability to mount an adequate immune response may be impaired.

Viral infections cause the majority of community-acquired fevers: influenza A and B (typically producing fever above 38.5°C with myalgia, headache, and dry cough); COVID-19 (SARS-CoV-2, with variable severity from mild febrile illness to severe pneumonia); Epstein-Barr virus (infectious mononucleosis — high fever, exudative pharyngitis, cervical lymphadenopathy, and splenomegaly in adolescents and young adults); rhinovirus (common cold — usually produces only low-grade fever); and COVID and influenza co-infections in winter months. Bacterial infections are the most important fever causes to identify as they require antibiotic treatment and can progress rapidly: urinary tract infection and pyelonephritis (bacteriuria with systemic features); community-acquired pneumonia (consolidative lung infection with fever, productive cough, dyspnoea); group A streptococcal pharyngitis (high fever, severe sore throat, tonsillar exudate — requires penicillin); skin and soft tissue infections including cellulitis (unilateral limb erythema, warmth, swelling with fever); and Salmonella typhi (enteric fever after travel to endemic regions — high sustained fever with relative bradycardia and rose spots). Malaria presents with cyclical fever, chills, and rigors after travel to endemic regions — always test for malaria in febrile returned travellers. Drug fever occurs 7–10 days after starting a new medication — commonly beta-lactam antibiotics, anticonvulsants, and allopurinol. Autoimmune and inflammatory causes of fever include adult-onset Still's disease, systemic lupus erythematosus, inflammatory bowel disease, and vasculitis. Fever of unknown origin (FUO) — fever above 38.3°C on three or more occasions over at least 3 weeks without identified cause — requires specialist systematic investigation.

Call emergency services immediately for fever with any of the following features that indicate potentially life-threatening infection. A non-blanching petechial or purpuric rash — one that does not fade when a glass is pressed against it — combined with fever is a medical emergency suggesting meningococcal septicaemia, which can be fatal within hours without immediate parenteral antibiotics (benzylpenicillin can be given by a GP before hospital transfer). Fever with severe headache and neck stiffness (inability to touch chin to chest without pain) and/or photophobia (aversion to light) indicates meningitis — a neurological emergency. Fever with confusion, disorientation, or altered level of consciousness suggests sepsis with cerebral involvement requiring immediate emergency treatment. Fever in any infant under 3 months old (under 12 weeks) must be treated as a potential serious bacterial infection and requires emergency evaluation — these infants cannot mount an adequate immune response and deteriorate rapidly. Fever with rapid progressive breathlessness, oxygen desaturation, or inability to speak in full sentences suggests severe pneumonia, pulmonary embolism, or acute respiratory distress syndrome. Fever with severe rigors (uncontrollable shaking chills) following travel to a malaria-endemic region requires same-day malaria testing — cerebral malaria from Plasmodium falciparum is fatal without treatment. Fever with a new heart murmur and recent dental procedure raises concern for infective endocarditis.

Accurate temperature measurement is the starting point: rectal thermometry is most precise and is recommended for infants and young children; digital oral thermometers provide reliable measurements in adults; tympanic (ear) thermometers are convenient but slightly less accurate; axillary measurement consistently underestimates core temperature by 0.5–1°C and should not be relied upon for clinical decisions. Detailed clinical history covers associated symptoms (cough, dysuria, sore throat, rash, diarrhoea, joint pain), duration and pattern of fever, travel history (especially to malaria, typhoid, or dengue-endemic regions within the past 3 months), immunisation history, medication history, sick contacts, and occupational exposures. Comprehensive physical examination specifically targets: skin for rashes (petechiae, purpura, erythema, urticaria) and evidence of cellulitis; oropharynx for exudate and tonsillar enlargement; lymph nodes; lung auscultation; cardiac auscultation for murmurs; abdomen for organomegaly and tenderness; joints for swelling and warmth; and neurological status including meningism. Laboratory investigations include: full blood count with differential (neutrophilia suggests bacterial infection; lymphocytosis suggests viral; eosinophilia suggests parasitic or drug reaction); CRP and procalcitonin (procalcitonin above 0.5 ng/mL strongly suggests bacterial infection and guides antibiotic decisions); blood cultures in triplicate before antibiotics in all patients with suspected sepsis; urine dipstick and culture; chest X-ray when pneumonia is suspected; and thick and thin blood films plus rapid antigen test for malaria in febrile returned travellers. Lumbar puncture is performed to obtain cerebrospinal fluid (CSF) for culture, sensitivity, cell count, protein, and glucose when meningitis or encephalitis is clinically suspected.

Antipyretic medications reduce fever, relieve associated discomfort (headache, myalgia), and improve the patient's sense of wellbeing. Paracetamol (acetaminophen) 500–1000 mg every 4–6 hours (maximum 4g in 24 hours in adults) is the safest first-line antipyretic — safe in pregnancy, hepatic disease at standard doses, and all ages above 3 months. Ibuprofen 400 mg every 6–8 hours (with food) is an effective alternative and combined analgesic-antipyretic in adults and children over 3 months — avoid in those with renal impairment, active peptic ulcer, asthma with known NSAID sensitivity, or in the third trimester of pregnancy. Paracetamol and ibuprofen can be alternated every 3–4 hours when one agent alone provides insufficient fever control. Maintain adequate hydration by increasing fluid intake — fever increases insensible water losses through sweating and tachypnoea, and dehydration is a common complication. Cool the environment and remove excess clothing; tepid sponging is no longer routinely recommended as it causes shivering, which generates heat. Antibiotics are prescribed only when a bacterial infection has been confirmed or is clinically strongly suspected based on localising signs, inflammatory markers, and culture results — the specific antibiotic and duration depend on the identified or likely pathogen, severity of illness, and local resistance patterns. Oseltamivir (Tamiflu) is effective for influenza in high-risk patients when started within 48 hours of symptom onset. Antimalarials (artemisinin-based combination therapy for P. falciparum; chloroquine for sensitive P. vivax) treat confirmed malaria. Aspirin must not be given to children under 16 due to the risk of Reye's syndrome.

Vaccination is the most powerful and cost-effective prevention for many of the most important fever-causing infections. Maintain up-to-date status for: annual influenza vaccination (particularly for over 65s, healthcare workers, pregnant women, and those with chronic lung, heart, or kidney disease); COVID-19 boosters as recommended; pneumococcal vaccination (PCV13 or PPSV23 for adults over 65 or those with chronic illness); meningococcal ACWY and B vaccines (adolescents and those in high-risk environments); Td/Tdap booster every 10 years for tetanus protection; and typhoid, hepatitis A, yellow fever, meningococcal, malaria prophylaxis, and Japanese encephalitis vaccines as appropriate for international travel destinations. Practise rigorous hand hygiene — thorough handwashing with soap and water for at least 20 seconds, particularly after using the toilet, before food preparation, after contact with animals, and after contact with sick individuals. Use alcohol-based hand gel when hand washing is unavailable. Avoid close contact with individuals with active febrile illness when possible. Use insect repellent (DEET-containing preparations) and permethrin-treated bed nets and clothing in malaria-endemic regions. Follow safe food handling principles to prevent enteric fever-causing infections from Salmonella and Campylobacter. In endemic regions, ensure drinking water is safe — boil, filter, or use bottled water. Maintain a healthy immune system through regular exercise, adequate sleep, nutritional sufficiency, and not smoking.

Call emergency services immediately for fever with a non-blanching rash (possible meningococcal septicaemia), fever with neck stiffness and severe headache (possible meningitis), fever with confusion or loss of consciousness, fever with severe progressive breathing difficulty, fever with signs of shock (rapid weak pulse, cold extremities, collapse), or any fever in an infant under 3 months old. Attend the emergency department for fever above 40°C (104°F) that is not responding to antipyretics, fever with rigors following travel to a malaria-endemic region, fever with a new heart murmur suggesting possible endocarditis, or fever in an immunocompromised patient (chemotherapy, transplant, HIV, on corticosteroids) which is always a potential emergency. Seek urgent same-day GP assessment for fever persisting more than 3 days in adults without an obvious cause, fever with localising signs such as urinary symptoms (UTI or pyelonephritis), earache, throat swelling, skin redness spreading rapidly (cellulitis), or severe unilateral facial pain (sinusitis or dental abscess). Schedule a GP appointment for fever that has resolved but left you with persistent fatigue, or for recurrent fevers without a clear explanation. Never self-treat fever in a returned international traveller without malaria testing first, as cerebral malaria progresses rapidly and can be fatal without treatment.