Definition and epidemiology
Fever is defined as a core body temperature typically above 38.0C (100.4F), though exact thresholds vary by measurement site and guideline. Oral, tympanic, and axillary measurements read lower than rectal or core (esophageal/bladder) temperatures, which are considered most accurate.
Normal body temperature follows a circadian rhythm, lowest in early morning and highest in late afternoon/evening, with a normal range of roughly 36.1-37.2C (97-99F). Fever reflects an upward shift of the hypothalamic set point, not simply a high number.
Fever is one of the most common presenting complaints in both outpatient and emergency settings, across all age groups. In children under 3 years it is especially frequent and usually due to self-limited viral illness. In older adults and immunocompromised patients, fever is less robust (blunted response) but still signals a higher likelihood of serious infection when present.
Fever of unknown origin (FUO) is classically defined as temperature >=38.3C on multiple occasions, lasting more than 3 weeks, with no diagnosis despite one week of inpatient (or equivalent outpatient) investigation. Infections, malignancies (especially lymphoma), and autoimmune/rheumatologic diseases account for most classic FUO cases; a substantial minority remain undiagnosed.
- Fever = regulated rise in hypothalamic set point via IL-1, IL-6, TNF-alpha, and PGE2; hyperthermia = normal set point with failed heat dissipation
- Antipyretics work by inhibiting COX/PGE2 synthesis in the hypothalamus; they do not work for true hyperthermia
- Classic FUO definition: >=38.3C, >3 weeks duration, no diagnosis after one week of appropriate inpatient workup
- Infants under 3 months with fever require urgent evaluation for serious bacterial infection
- Neutropenic fever is a medical emergency requiring immediate empiric broad-spectrum antibiotics
- Aspirin is avoided in febrile children/adolescents due to Reye syndrome risk
- Relative bradycardia with fever suggests typhoid fever, drug fever, or Legionella (Faget sign)
- Pel-Ebstein fever pattern is classically associated with Hodgkin lymphoma
Etiology and risk factors
Infectious (most common overall)
- Viral upper and lower respiratory infections
- Bacterial infections: pneumonia, urinary tract infection, skin/soft tissue infection, intra-abdominal infection, endocarditis
- Tuberculosis, especially in endemic areas or immunocompromised hosts
- Malaria and other travel-related infections in returning travelers
- Sepsis and bloodstream infections
Non-infectious
- Autoimmune/rheumatologic disease: adult-onset Still disease, systemic lupus erythematosus, vasculitides, giant cell arteritis/polymyalgia rheumatica
- Malignancy: lymphoma, leukemia, renal cell carcinoma, hepatocellular carcinoma (tumor or paraneoplastic cytokine-mediated fever)
- Drug fever (antibiotics, anticonvulsants, allopurinol)
- Venous thromboembolism
- Postoperative/tissue injury (within 48-72 hours)
- Endocrine: thyroid storm
- Central causes: rare hypothalamic lesions
Risk factors for serious underlying illness
- Age under 3 months or over 65 years
- Immunosuppression (HIV, chemotherapy, transplant, chronic steroids)
- Recent travel, especially to malaria-endemic regions
- Indwelling devices or recent surgery
- IV drug use (endocarditis risk)
- Underlying chronic disease (diabetes, cirrhosis, asplenia)
Pathophysiology
Exogenous pyrogens (microbial products such as lipopolysaccharide) and tissue damage trigger macrophages and other immune cells to release endogenous pyrogens, primarily IL-1, IL-6, and TNF-alpha. These cytokines act on the vascular endothelium of the hypothalamic organum vasculosum, inducing cyclooxygenase-2 (COX-2)-mediated synthesis of prostaglandin E2 (PGE2).
PGE2 binds EP3 receptors on hypothalamic neurons, raising the thermoregulatory set point. The body then generates heat through:
- Peripheral vasoconstriction (reduces heat loss)
- Shivering (increases heat production)
- Behavioral changes (seeking warmth, chills)
Once the set point returns to normal (infection controlled, pyrogen cleared, or antipyretic given), vasodilation and sweating dissipate the excess heat.
This mechanism explains why antipyretics (acetaminophen, NSAIDs) work by inhibiting COX and reducing PGE2 synthesis, lowering the set point, whereas external cooling alone is ineffective for true fever because the hypothalamus will simply drive further heat generation to defend the elevated set point. This is the key distinction from hyperthermia, where the set point is normal but heat gain exceeds dissipation (heat stroke, malignant hyperthermia, serotonin syndrome, neuroleptic malignant syndrome) — these conditions require active cooling and do not reliably respond to antipyretics.
Clinical features
Fever itself is a sign, not a disease, so clinical evaluation focuses on associated symptoms and pattern.
General and constitutional
- Chills and rigors (suggest bacteremia or significant pyrogen surge)
- Malaise, myalgias, headache
- Diaphoresis during defervescence
- Tachycardia and tachypnea (roughly proportional rise with temperature)
Fever patterns (classic but low specificity)
- Continuous/sustained: typhoid fever, brucellosis
- Intermittent: fluctuates to normal daily; seen in pyogenic infections, malaria
- Remittent: fluctuates but never returns to baseline; seen in infective endocarditis
- Tertian/quartan: cyclical patterns classically described in malaria (Plasmodium vivax/ovale = tertian, P. malariae = quartan)
- Pel-Ebstein: cyclical fever historically associated with Hodgkin lymphoma
Red flag ('alarm') features warranting urgent evaluation
- Hypotension, altered mental status, or signs of sepsis
- Rash (especially petechial/purpuric), neck stiffness, photophobia (meningitis)
- Severe localized pain (abdomen, flank, chest)
- Neutropenia or known immunosuppression
- Age under 3 months with any fever
- New murmur or embolic phenomena (endocarditis)
Diagnosis
Diagnosis of the cause of fever is directed by history and exam rather than a fixed panel.
History
- Onset, duration, pattern, height of fever
- Associated localizing symptoms (cough, dysuria, headache, rash, joint pain)
- Travel, sick contacts, animal/vector exposure
- Immunization and immune status
- Medication history (drug fever)
- Recent procedures, devices, surgery
Examination
- Full systems review looking for a focal source
- Skin, lymph nodes, cardiac auscultation (murmurs), joints
- Signs of sepsis (hypotension, tachycardia, altered mentation)
Initial investigations (guided by presentation)
- Complete blood count with differential
- Blood cultures (before antibiotics, if infection suspected and patient is unwell)
- Urinalysis and urine culture
- C-reactive protein and/or procalcitonin
- Chest radiograph if respiratory symptoms
- Site-specific cultures/imaging as indicated
Fever of unknown origin workup
If no source is found after initial evaluation and fever persists:
- Repeat and broaden cultures
- HIV testing, tuberculosis evaluation (interferon-gamma release assay or tuberculin skin test, chest imaging)
- Autoimmune serologies (ANA, rheumatoid factor, ESR/CRP)
- CT chest/abdomen/pelvis; consider PET-CT for occult malignancy or vasculitis
- Echocardiography if endocarditis suspected
- Bone marrow biopsy or lymph node biopsy if hematologic malignancy suspected
| Temperature site | Typical offset vs core |
|---|---|
| Rectal/bladder (core) | Reference standard |
| Oral | ~0.3-0.5C lower |
| Axillary | ~0.5-1.0C lower |
| Tympanic | Variable, user-dependent |
Differential diagnosis
The differential for fever is broad; narrowing it depends on associated findings.
- Hyperthermia syndromes (heat stroke, malignant hyperthermia, neuroleptic malignant syndrome, serotonin syndrome) — normal set point, do not respond to antipyretics, require active cooling
- Sepsis/bacteremia — hemodynamic instability, elevated lactate
- Viral syndromes — self-limited, often with coryza, myalgias
- Drug fever — temporal relation to new medication, relative bradycardia, resolves on discontinuation
- Malignancy-associated fever — lymphoma, renal cell carcinoma, often with weight loss and night sweats
- Autoimmune disease — adult-onset Still disease (fever, arthralgia, salmon-colored rash), SLE, vasculitis
- Thyroid storm — fever with tachycardia, tremor, agitation, history of hyperthyroidism
- Factitious fever — discordance between measured temperature and clinical appearance; normal diurnal variation absent
Management
Management is directed primarily at the underlying cause; fever reduction itself is mainly for symptomatic relief, except in specific high-risk situations.
General principles
- Treat the identified source (antibiotics for bacterial infection, antivirals where indicated, source control for abscess, etc.)
- Antipyretics (acetaminophen/paracetamol or NSAIDs such as ibuprofen) reduce discomfort but do not shorten illness duration and are not required solely to "treat" fever in most otherwise well patients
- Avoid routine combination or alternating antipyretic regimens in children; most guidelines (WHO, NICE, AAP) favor single-agent therapy for comfort, with alternating/combination therapy used cautiously if at all
- Aspirin is avoided in children and adolescents with febrile viral illness due to Reye syndrome risk
- Adequate hydration and comfort measures (light clothing, ambient cooling)
- Physical cooling methods (tepid sponging) have limited evidence for true fever and are not routinely recommended as primary therapy, since the hypothalamic set point will counteract passive cooling
When fever itself needs active treatment
- Febrile patients with limited cardiopulmonary reserve (severe heart failure, COPD) because increased metabolic demand can precipitate decompensation
- Pregnant patients, where guidelines generally favor treating fever (acetaminophen preferred) due to theoretical risk of neural tube defects with first-trimester hyperthermia
- Febrile seizure history in young children — antipyretics improve comfort though do not reliably prevent seizure recurrence
Special populations
- Infants under 3 months: fever is managed as a marker of potential serious bacterial infection; most guidelines recommend urgent evaluation and often empiric workup/admission
- Neutropenic fever (oncology patients): medical emergency, requires immediate blood cultures and empiric broad-spectrum antibiotics, per institutional/IDSA protocols, without waiting for antipyretic response
- Returning travelers: malaria must be excluded urgently given risk of rapid deterioration with P. falciparum
Guideline notes
WHO, NICE, and the American Academy of Pediatrics converge on treating the child, not the number: antipyretics are for comfort, not to normalize a specific temperature target, and should not be used solely to prevent febrile seizures.
Complications and prognosis
Most fever, especially from self-limited viral illness, resolves without complication.
Potential complications
- Febrile seizures in young children (typically 6 months to 5 years), usually benign and self-limited
- Dehydration from increased insensible losses and reduced intake
- Decompensation of underlying cardiopulmonary or metabolic disease
- Delayed recognition of serious bacterial infection or sepsis if fever is dismissed as benign
- Progression to severe sepsis/septic shock if the underlying infectious cause is untreated
Prognosis
Prognosis depends entirely on the underlying etiology rather than the fever itself. Fever from a viral upper respiratory infection typically resolves within days. Fever from untreated bacterial sepsis carries significant morbidity and mortality if not promptly treated. In fever of unknown origin, prognosis varies: infectious causes are often curable, autoimmune causes are usually manageable, and malignancy-associated FUO carries a prognosis dependent on the specific cancer and stage. A meaningful minority of FUO cases resolve spontaneously without ever reaching a diagnosis.
Causes of FUO - 'MAID': Malignancy, Autoimmune disease, Infection, Drug fever (plus consider factitious fever if none fit).
Practice questions
1. A 32-year-old man develops a temperature of 39.2C three days after starting a new antibiotic for a dental abscess. He feels well otherwise, has a heart rate of 78/min (lower than expected for the degree of fever), and the abscess has clinically improved. Which of the following is the most likely explanation?
2. A 68-year-old woman with lymphoma presents with fevers up to 39.5C that have occurred in a cyclical pattern over several weeks, alternating with afebrile periods of several days, along with weight loss and night sweats. Which classic fever pattern is being described?
3. A 6-week-old infant is brought in with a rectal temperature of 38.4C and no clear source on exam. The infant is feeding well and appears well. Which of the following is the most appropriate next step?
Frequently asked questions
What causes fever?
Fever is caused by pyrogens, usually from infection, that trigger release of cytokines such as IL-1, IL-6, and TNF-alpha, which raise the hypothalamic set point via prostaglandin E2. Common causes include viral and bacterial infections, autoimmune disease, malignancy, and certain drugs. The body then generates heat through vasoconstriction and shivering to reach the new, higher set point.
What are the first symptoms of fever?
The first symptoms are usually chills, shivering, and a feeling of being cold as the body raises its temperature to the new set point, followed by malaise, headache, and muscle aches. As the fever peaks, patients may feel hot and sweaty during defervescence. Associated symptoms depend on the underlying cause, such as cough, sore throat, or urinary symptoms.
How is fever diagnosed?
Fever is diagnosed by measuring core or near-core body temperature above approximately 38.0C (100.4F), most accurately via rectal, bladder, or esophageal probes. The real diagnostic work is finding the cause, through history, physical exam, and targeted tests such as blood counts, cultures, urinalysis, and imaging based on localizing symptoms. If no cause is found after extensive evaluation and fever persists beyond three weeks, it is classified as fever of unknown origin.
Is fever dangerous?
Fever itself is usually not dangerous and is a normal immune response, but very high or prolonged fever can cause dehydration, febrile seizures in young children, and strain on the heart and lungs in vulnerable patients. The main danger is usually the underlying cause rather than the fever itself. Fever in infants under 3 months, immunocompromised patients, or with signs of sepsis warrants urgent medical attention.
Fever vs hyperthermia — what is the difference?
Fever is a regulated increase in the hypothalamic set point driven by pyrogens and cytokines, so the body actively generates heat to reach a new, higher target. Hyperthermia is an unregulated rise in temperature with a normal set point, caused by excess heat production or failed heat dissipation, as in heat stroke or malignant hyperthermia. This distinction matters clinically because antipyretics work for fever but not for hyperthermia, which requires active cooling.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: R50
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.