On March 31, 2013, Chinese health officials notified the World Health Organization of three cases of human infection with novel influenza A (H7N9). Since then, 132 people have been infected, 37 of them fatally (see figure, Panel A). To date, there is no evidence of ongoing human-to-human transmission. However, a number of characteristics of this virus are cause for heightened attention. First, the human population has not been exposed on a large scale to hemagglutinin 7 (H7) and neuraminidase 9 (N9) antigens in recent times and therefore most likely lacks immunity against this strain. Second, initial analyses of viral genome . . .