This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Virology, Subject name: Microbiology
Which influenza strain has caused pandemic 
Select and explain. 
A. H1N1, B. H2N2, C. H5N14, D. H9N1
A: H1N1
Answer: A: H1N1 
Explanation: 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