The flu is caused by influenza virus types A, B, and C. Both type A and type B flu viruses are responsible for the seasonal outbreaks of flu. Type A flu viruses are found in many different animals, including ducks, chickens, pigs, and horses. Influenza B viruses circulate widely only among humans.
Flu viruses spread mainly from person to person, through coughing or sneezing by people infected with influenza. Sometimes people may become infected by touching a contaminated surface and then touching their mouth or nose. Healthy adults may be able to infect others beginning one day before symptoms develop and up to seven days after becoming sick. This means a person may be able to pass on the flu to someone else before ever knowing they are sick.
