Influenza

Influenza is an acute viral respiratory illness caused by influenza virus types A, B, and C. In humans, influenza A and B cause seasonal epidemics; influenza C typically causes milder illness. The viruses evolve through antigenic drift, and influenza A can also undergo antigenic shift, which can lead to pandemics.

Influenza spreads mainly through respiratory droplets and aerosols, and also by touching contaminated surfaces. The incubation period is about one to four days. Most people recover within a week, but fever, cough, sore throat, fatigue, and body aches are common. Severe disease can occur, especially in young children, older adults, pregnant people, and those with chronic conditions.

Diagnosis is based on clinical presentation during outbreaks and is confirmed by laboratory testing when needed. Rapid antigen tests are available but vary in sensitivity; reverse transcription polymerase chain reaction (PCR) testing is more accurate and is commonly used in healthcare settings.

Prevention relies primarily on annual vaccination, which is reformulated each year to match circulating strains. Vaccination reduces the risk of illness, hospitalization, and death. Treatments include antiviral medications such as neuraminidase inhibitors (oseltamivir, zanamivir) or baloxavir, most effective when started within 48 hours of symptom onset. Supportive care is important for most cases.

Globally, seasonal influenza causes substantial morbidity and mortality, with winter peaks in temperate regions. Antigenic drift drives yearly epidemics, while antigenic shift in animal reservoirs can trigger pandemics. Notable pandemics occurred in 1918, 1957, 1968, and 2009. Public health surveillance and vaccination programs continue to reduce impact and guide response strategies.