Seasonal influenza is the most common infection and usually causes a self-limited febrile illness with upper respiratory symptoms and malaise that resolves within 10 days.

Influenza occurs all over the world, with an annual global attack rate estimated at 5–10 per cent in adults and 20–30 per cent in children. In temperate regions, influenza is a seasonal disease occurring typically in winter months: it affects the northern hemisphere from November to April and the southern hemisphere from April to September. In tropical areas there is no clear seasonal pattern, and influenza circulation is year-round, typically with several peaks during rainy seasons. Influenza remains one of the world’s greatest public health challenges. Every year across the globe, there are an estimated 1 billion cases, of which 3 to 5 million are severe cases, resulting in 290 000 to 650 000 influenza-related respiratory deaths.

Seasonal influenza is a viral respiratory infection that affects the nose, throat, bronchi and, occasionally, the lungs. Infection usually lasts for about a week and is characterized by sudden onset of high fever, aching muscles and joints, headache and severe malaise, non-productive cough, sore throat and rhinitis. Seasonal influenza is caused by human types of influenza viruses – influenza B, C and few sub-types of A – which circulate worldwide and can affect people in any age group. They are transmitted easily from person to person via droplets and small particles produced when infected people cough or sneeze. The time from infection to illness is about 2 days. Most infected people recover within 1 to 2 weeks without requiring medical treatment.

However, in high risk groups, including the very young, the elderly, and those with serious medical conditions, infection can lead to severe complications of the underlying condition, pneumonia and death. To prevent transmission, people should cover their mouth and nose when coughing and wash their hands regularly. Influenza tends to spread rapidly in seasonal epidemics. Influenza epidemics can take an economic toll through lost workforce productivity and strained health services.