H7N9 Influenza: What We Learned from H5N1: The H7N9 flu that emerged in 2013 is 30% lethal in humans but, unlike H5N1, causes no apparent illness in birds

During the past year, a second bird flu emerged in Asia—an avian influenza virus that is sporadically transmitted to humans. This virus of the H7N9 subtype proved lethal in 33% of the 135 cases reported as of October 2013, yet it is termed a “low-pathogenic” virus because of the mild to inapparent respiratory disease it causes in domestic poultry. In contrast, the H5N1 influenza virus that emerged in Southern China in 1997 is highly pathogenic to poultry, killing as many as 100% of the birds it infects. It, too, is sporadically transmitted to humans, and is lethal in about 60% of reported cases.