Figure 3.

Reconstruction of the sequence of reassortment events leading up to the emergence of pandemic H1N1/09 virus. Shaded boxes represent host species: avian (green), swine (orange), and human (purple). Colored lines represent interspecies‐transmission pathways of influenza genome segments. The eight genomic segments are represented as parallel lines in descending order of size. Dates marked with dashed vertical lines on “elbows” indicate the mean time of divergence of the pandemic H1N1/09 segments from corresponding virus lineages. Reassortment events not involved with the emergence of human disease are omitted. Fort Dix refers to the last major outbreak of swine‐derived influenza H1N1 in humans. The first triple‐reassortant swine viruses were detected in 1998, but, to improve clarity, the origin of this lineage is placed earlier. Figure reprinted from Ref. 147.