China reported its first human infections with avian influenza A(H7N9) virus in late March 2013 . In the following weeks, 131 human infections were confirmed; 33 occurred in Shanghai. Because infection with this novel virus had a high fatality rate and posed a pandemic risk, Shanghai disease control authorities launched rapid investigations to identify the source of the infections. Migratory birds, mammals, poultry, and humans could be potential reservoirs of H7 subtype avian influenza viruses, so all of these possibilities were simultaneously evaluated immediately after the discovery of the novel virus.