Popularly known as “the flu,” influenza is a highly contagious disease of the airways caused by the influenza virus. Immunization is possible by influenza vaccination, which bestows protection for up to one year. Each year, influenza claims roughly 500,000 lives. The most recent large-scale pandemics include the Spanish Flu (1918-1920), the Asian Flu (1957-1958), and the Hong Kong Flu (1968-1970). See also avian flu. The term “flu” is sometimes used colloquially to describe the common cold.