Which influenza strain has caused pandemic

Epidemiology of H1N1 Origin: H1N1 2009 flu originated by genetic reassoment of four strains and the mixing had occured in pigs Transmission: it can be transmitted from human to human, which has accounted for its rapid spread. Currently, world is in the post-pandemic period except in India and New Zealand where still local intense transmission is going on Ref : essentials of Medical microbiology apurba sastry 1st edition page 463