Influenza

Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. The etiologic agents belong to the family Orthomyxoviridae and comprise four distinct types: A, B, C, and D. Influenza A virus is the primary driver of severe illness and seasonal epidemics, with aquatic birds serving as the natural reservoir. Influenza B and C viruses predominantly infect humans, while influenza D virus circulates in cattle and pigs. The viruses possess a negative-sense, single-stranded RNA genome that is segmented, facilitating genetic reassortment and viral evolution.

Influenza presents with a spectrum of illness ranging from mild to severe, typically beginning one to four days after exposure and lasting approximately two to eight days. Core symptoms include fever, headache, muscle pain, fatigue, cough, sore throat, and runny nose, with sudden onset of initial non-specific symptoms including chills, malaise, and loss of appetite. Gastrointestinal manifestations such as diarrhea and vomiting occur more frequently in children. Complications include primary viral pneumonia, secondary bacterial pneumonia, acute respiratory distress syndrome, meningitis, and encephalitis. The infection may also exacerbate underlying chronic conditions including asthma and cardiovascular disease.

Influenza A and B viruses circulate globally and cause seasonal epidemics, typically during winter months in temperate zones. The disease burden is substantial, with annual outbreaks affecting all age groups but causing disproportionate morbidity and mortality in vulnerable populations. Influenza C virus causes mild respiratory illness primarily in children, while influenza D virus has not been associated with human disease. The virus's segmented genome enables antigenic shift, which can generate novel strains with pandemic potential when introduced into immunologically naive human populations. Antigenic drift, the gradual accumulation of mutations in surface proteins hemagglutinin and neuraminidase, contributes to annual epidemic variability and necessitates regular vaccine reformulation.

Human-to-human transmission occurs primarily through respiratory droplets generated by coughing and sneezing, which can deposit on the mucous membranes of susceptible individuals in close proximity. Airborne transmission via aerosols and fomite transmission through contaminated surfaces also contribute to spread. The virus binds to cells containing sialic acid receptors via viral hemagglutinin proteins, with neuraminidase facilitating viral release and spread. Frequent hand washing and respiratory etiquette such as covering the mouth and nose during coughs are recognized preventive measures.

Source-backed detail on specific high-risk groups is not yet available in the provided source material. However, influenza A virus is documented to cause severe illness disproportionately in certain populations, and the disease may progress to complications in individuals with pre-existing health conditions including asthma and cardiovascular disease.

Annual vaccination is the primary strategy for influenza prevention, though specific vaccine composition and scheduling details are not provided in available source material. Non-pharmaceutical interventions include frequent hand washing and covering the mouth and nose during respiratory events. The effectiveness of vaccination depends on the antigenic match between circulating strains and vaccine components, which is complicated by ongoing antigenic drift. Population-level immunity through vaccination reduces transmission and protects vulnerable individuals through herd effects.

Influenza surveillance systems monitor circulating strains, antiviral resistance patterns, and disease activity to inform public health response. The virus's capacity for antigenic evolution through drift and shift requires continuous virological surveillance to detect emerging variants and update vaccine strains. Laboratory-based surveillance tracks the proportion of influenza-like illness attributable to influenza viruses and characterizes circulating subtypes and lineages. Integration of epidemiological and virological data enables early detection of antigenic drift events that may reduce vaccine effectiveness.

1. 1 WHO Fact Sheet. Detail.
2. 2 WHO Q&A. News.
3. 3 Wikidata. influenza.
4. 4 Wikipedia. Influenza - Wikipedia.

ICD-10

J09-J11

ICD-11

1E30

Total cases

72.7M

Total deaths

951

Peak month

2025-12

Coverage

3 reporting countries · 2000-01-01 → 2026-05-02