Fig. 1.

Timeline of Influenza A serotype emergence by year and zoonotic host from 1918 to 2015. Each point indicates a distinct avian influenza serotype known to have caused human infection, placement of point corresponds to their year of emergence. The different point shapes correspond to the animal host which was identified when the serotype was first reported in humans: a pentagon corresponds to a seal host, a triangle corresponds to avian host, a cross corresponds to swine host, and a circle corresponds to instances where the animal host has not yet been identified