Peter Palese, Professor and Chairman of the Department of Microbiology and Infectious Diseases at Mt. Sinai, explains the current Viruss direct derivation from that which arose in 1918, the natural herd immunity that all humans share against it, and the reasons why the elderly stand at a lesser risk of contracting the contagion:

http://bigthink.com/peterpalese/the-wiley-influenza-virus-porcine-and-otherwise.