Influenza (the flu) is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death. Each year in the United States on average, 5% to 20% of the population gets the flu; on average, more than 200,000 people are hospitalized from flu-related complications; and about 36,000 people die from flu-related causes. Some people, such as older people, young children, and people with certain health conditions, are at high risk for serious flu complications.
Every year, seasonal influenza affects 5-15% of the population in the northern hemisphere. Many of these people consult a doctor. Most of them do not need medical treatment, but 3-5 million infections cause severe disease resulting in hospitalization or even death. Influenza therefore significantly affects not only the public but also health systems and the economy.
Types of virus
There are two main types of influenza (flu) virus: Types A and B. Influenza A and B viruses that routinely spread in people (human influenza viruses) are responsible for seasonal flu epidemics each year. The emergence of a new influenza virus causing illness in people can result in an influenza pandemic, as occurred in the spring of 2009 when a new influenza A (H1N1) virus emerged to cause illness in people and spread worldwide. Influenza A viruses can be broken down into sub-types. Over the course of a flu season, different types (A & B) and subtypes of influenza A viruses can circulate and cause illness. In addition, influenza viruses are constantly changing through a process called "antigenic drift."
Pandemic influenza
An influenza pandemic occurs when a new influenza virus subtype emerges and spreads easily among human beings. Pandemics have been observed for several hundreds years. The best-documented pandemics occurred during the 20th century, in 1918 (Spanish influenza), 1957 (Asian influenza) and 1968 (Hong Kong influenza). These varied in severity, with an estimated 1-50 million excess deaths during the pandemic.
