Influenza is an acute viral respiratory tract illness, caused by one of three influenza types (A, B and C), of which types A and B are most commonly associated with infections in humans.1
Influenza is highly infectious and spreads rapidly, particularly in close communities such as care homes and schools. Transmission is via droplet spread, usually through symptomatic individuals coughing or sneezing. People with mild or no symptoms can still infect others. The usual incubation period is one to three days.
Symptoms are usually self-limiting, and include sudden-onset fever, myalgia, cough, headache and malaise.1
Severe flu complications are more common in individuals who are less immuno-
competent, including the very young and the elderly. Pregnant women, and those with underlying long-term health conditions (such as diabetes, chronic respiratory disease, and cardiac disease) are also prone to severe complications.1
Influenza viruses have been responsible for significant outbreaks and pandemics for several centuries, including the well-documented global pandemic of 1918/19, which caused an estimated 21 million deaths.