Conditions · Infectious and tropical diseases
Influenza (flu) !
Influenza is a contagious viral infection of the airways with sudden fever and aches. It can be severe in older adults, small children and pregnant women.
Updated 2026-11-01
Structures involved
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Microorganisms that cause it
Why it happens
- Systemic effects of inflammation: fever, acute-phase proteins, leukocytosis, sepsis
- Airway defenses, cough and mucociliary clearance
- Fever
- The airway: from nose to bronchioles
- Cilia, microvilli and stereocilia
What it is
An acute respiratory infection caused by influenza A and B viruses, with sudden fever, myalgia and cough. It occurs in seasonal epidemics and, when a new influenza A subtype appears, in pandemics.
Who gets it
Causes about a billion infections and hundreds of thousands of respiratory deaths worldwide each year. In tropical countries such as Colombia it circulates all year with peaks during the rainy seasons; it is monitored by the national surveillance of influenza-like illness and severe acute respiratory infection (IRAG). Seasonal vaccination is part of the national programme for priority groups.
Causes
- Influenza A virus (subtypes by haemagglutinin and neuraminidase, e.g. H1N1, H3N2) and influenza B virus
- Spread by respiratory droplets, aerosols and contaminated hands or surfaces
- Antigenic drift (small mutations) causes yearly epidemics; antigenic shift (gene reassortment) can cause pandemics
Risk factors
- Age under 5 years (especially under 2) and 65 years or older
- Pregnancy and up to two weeks after delivery
- Chronic heart, lung, kidney, liver or metabolic disease, including diabetes and obesity
- Immunosuppression
- Living in care homes and working in health care
Pathophysiology
The viral haemagglutinin binds sialic acid on respiratory epithelial cells, allowing entry; neuraminidase releases new virions. Infection of the upper and lower airway epithelium causes cell death, ciliary loss and inflammation, with systemic symptoms driven by cytokines. Damaged epithelium favours secondary bacterial pneumonia, classically by Streptococcus pneumoniae and Staphylococcus aureus.
Classification
- Uncomplicated influenza
- Complicated or severe influenza (pneumonia, respiratory failure, exacerbation of chronic disease)
- Seasonal versus pandemic influenza
Symptoms and signs
- Sudden onset of high fever and chills
- Myalgia, headache and marked malaise
- Dry cough, sore throat and runny nose
- Fatigue that can last weeks; cough may persist two weeks or more
- Vomiting and diarrhoea, more common in children; older adults may present without fever
Diagnosis
- Clinical
- Influenza-like illness (sudden fever with cough or sore throat) during periods of circulation; treatment in high-risk or severe cases should not wait for test results.
- Tests
- RT-PCR on nasopharyngeal swab (most sensitive, also used in surveillance); rapid antigen tests have lower sensitivity. Multiplex panels can also detect SARS-CoV-2 and RSV.
- Imaging
- Chest X-ray when pneumonia is suspected: viral interstitial infiltrates or a lobar consolidation if there is secondary bacterial infection.
Differential diagnosis
- COVID-19
- Respiratory syncytial virus and other respiratory viruses (common cold)
- Bacterial community-acquired pneumonia
- Dengue in endemic areas (fever, myalgia, headache)
- Streptococcal pharyngitis
Goal of treatment
Relieve symptoms, shorten illness and prevent complications and death in high-risk or severe cases, while limiting spread to others.
Treatment
- Conservative
- Rest, fluids and antipyretics for uncomplicated cases; aspirin is avoided in children and adolescents because of the risk of Reye syndrome.
- Medical
- Neuraminidase inhibitors (oseltamivir, zanamivir, peramivir) or the endonuclease inhibitor baloxavir, ideally started within 48 hours; given promptly to hospitalised, severe or high-risk patients regardless of timing.
- Medical
- Antibiotics only when secondary bacterial pneumonia is suspected or confirmed.
- Acute
- Hospital care for severe cases: oxygen, ventilatory support and management of sepsis or respiratory failure.
- Follow-up
- Droplet precautions and isolation in hospital, monitoring of respiratory rate and saturation, and notification of severe acute respiratory infection to surveillance.
Drugs for this condition
Complications
- Primary viral pneumonia and acute respiratory distress syndrome
- Secondary bacterial pneumonia (pneumococcus, Staphylococcus aureus)
- Exacerbation of asthma, COPD and heart failure; myocardial infarction
- Myocarditis, encephalitis and Guillain-Barré syndrome
- Otitis media and sinusitis in children; Reye syndrome with aspirin
Prognosis
Most people recover in about a week. Mortality concentrates in older adults, young children, pregnant women and people with chronic diseases, and is higher during pandemics.
Prevention
- Annual influenza vaccination, especially for pregnant women, children 6 to 23 months, older adults, people with chronic disease and health workers
- Hand hygiene, cough etiquette and staying home when ill
- Ventilation and use of masks in health facilities during circulation
- Antiviral chemoprophylaxis for selected exposed high-risk people
Test yourself
Think of the answer, then tap the question to see it.
What is the difference between antigenic drift and antigenic shift?
Drift is small point mutations causing seasonal epidemics; shift is reassortment of gene segments creating a new subtype that can cause pandemics.
How do neuraminidase inhibitors act?
They block viral neuraminidase, preventing release of new virions from infected cells.
Which bacteria classically cause pneumonia after influenza?
Streptococcus pneumoniae and Staphylococcus aureus (also Haemophilus influenzae).
Why is aspirin avoided in children with influenza?
Because of the risk of Reye syndrome: acute encephalopathy with liver failure.
Related conditions
- COVID-19
- Community-acquired pneumonia
- Acute respiratory distress syndrome (ARDS)
- Sepsis and septic shock
- Myocarditis
- Guillain-Barré syndrome
- Bronchiolitis (RSV)
- Acute otitis media
- Acute rhinosinusitis
- Dengue
- Streptococcal pharyngitis
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Influenza (seasonal) (World Health Organization (WHO), 2025).
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