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