A study pieces together the evolutionary events leading up to the first human case of avian-derived H7N9 influenza. Since the virus’s 2013 emergence in humans, it has been linked to two outbreaks and at least 115 deaths. Phylogenetic analysis suggests the virus is a triple reassortment of H7, N9, and H9N2 avian influenza viruses, but it remains unclear how the prevalence and evolution of these three viruses gave rise to novel H7N9.
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