<prompt> Swine flu is caused most commonly by-H1N1H5N1H3N2B virus  <response>: ref: parks text book 23rd edition Swine flu is caused by the pandemic influenza virus H1N1 Its incubation period varies from 2-3 days It differs in its pathogenicity from seasonal influenza in 2 ways: - majority of human population has little or no immunity against it,the impact of infection has been in a wider range. -virus can infect lower respiratory tract and cause rapidly progressive pneumonia,especially in children and young to middle aged adults. . 