Influenza is a contagious disease caused by a virus.
A virus is a germ that is very small. Influenza viruses infect many parts of the body, including the lungs. When someone who has influenza sneezes, coughs, or even talks, the influenza virus is expelled into the air and may be inhaled by anyone close by. When influenza strikes the lungs, the lining of the respiratory tract is damaged. The tissues become swollen and inflamed.