The most common types are influenza A and influenza B, which cause most of the flu outbreaks that typically occur in the late fall and winter months. Although people think of influenza as a disease that affects the whole body, it actually is site specific, meaning that it lives and multiplies primarily in one area of the body, the lungs.
Influenza is highly contagious, meaning that it passes very easily from one person to another.
That is why flu epidemics are common in many parts of the world. The flu virus is most often passed through coughing and sneezing, but it may also be passed through a handshake or other physical contact. Typically, when a flu-infected person coughs or sneezes, the droplets are inhaled by an uninfected person nearby, who then becomes infected. That's why influenza is called an airborne disease. Once a person has become infected with influenza, symptoms usually begin appearing 24 to 48 hours later.