If you use assistive technology (such as a screen reader) and need a
version of this document in a more accessible format, please email firstname.lastname@example.org.
Please tell us what format you need. It will help us if you say what assistive technology you use.
Influenza is an acute viral infection of the respiratory tract. There are 3 types of influenza virus: A, B and C. Influenza A and influenza B are responsible for most clinical illness. Influenza is highly infectious with a usual incubation period of 1 to 3 days.