Swine flu is caused most commonly by -

<p> SWINE FLU:- Pandemic influenza A (H1N1). This differs from seasonal influenza in two ways. 1. Majority of population has little/no pre existing immunity to virus,the impact of the infection thus leading to wider range. 2. The virus infect the lower respiratory tract and can cause progressive pneumonia. Incubation period:- 2-3 days but can range upto 7 days. {Reference: park&;s textbook of preventive and social medicine, 23rd edition, pg no.156}</p>