Evolution of Flu Viruses, 1918-2009
Genetic relationships among human and swine influenza viruses, 1918-2009. Red arrows indicate human influenza virus lineages, black arrows swine influenza virus lineages, and gray arrows exportation of one or more genes from the avian influenza A virus gene pool. Horizontal bars shown inside the virus represent each of the eight virus genes, abbreviated PB2, PB1, PA, HA, NP, NA, M and NS. Credit: NIAID
Read more about the origin of swine flu here.