<prompt> Swine flu is caused most commonly by -H1N1H5N1H3N2B virus  <response>: SWINE FLU:- Pandemic influenza A H1N1. This differs from seasonal influenza in two ways. 1. Majority of population has littleno 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 