In plain words
Seasonal influenza (the flu) is an acute respiratory infection caused by influenza viruses, found in all parts of the world. It spreads easily when people cough or sneeze. Most people recover without treatment, and vaccination is the best way to prevent it.
Key facts
- About a billion cases a year, including 3 to 5 million severe cases.
- 290,000 to 650,000 respiratory deaths a year.
- 99% of influenza-related lower respiratory deaths in children under 5 are in developing countries.
- Symptoms begin 1 to 4 days after infection and usually last about a week.
Symptoms
Sudden fever, cough, sore throat, body aches and fatigue.
Virus types
There are four types, A, B, C and D. A and B cause seasonal epidemics. Influenza A is divided into subtypes; A(H1N1) and A(H3N2) currently circulate in people. Influenza B has B/Yamagata and B/Victoria lineages. C usually causes mild infection, and D mainly affects cattle and is not known to infect people.
Treatment
Aimed at relieving symptoms: rest and plenty of fluids. Most people recover within a week. Severe cases and people with risk factors may need medical care.
Sources
- WHO fact sheet, Influenza, published 28 February 2025, read 9 October 2026.
Last reviewed: 9 October 2026. Reviewer: none yet. This page is general education and is not medical advice. Figures are WHO's and may have been updated since.