A 28-year-old man (index patient), with repeated exposure to live poultry, presented with respiratory infection and was admitted to the respiratory department, where his condition worsened. Laboratory investigation of serum and sputum samples obtained later in the course of illness showed that he was positive for H7N9 by serologic testing and polymerase-chain-reaction (PCR) assay. Seven days after admission of the index patient, influenza-like illness developed in a 33-year-old male physician (Doctor 1) who attended the index patient. The acute respiratory distress syndrome later developed in this physician. Four days after the onset of symptoms in Doctor 1, influenza-like illness and bronchial pneumonia developed in a second attending physician (Doctor 2), a 35-year-old man, in the same department (Figure 1AFigure 1Clinical Events and Phylogenetic Analysis of the H7N9 Influenza Cluster.). He too had close contact with the index patient. Although standard infection-control practices, including the wearing of personal protective equipment, are hospital policy when caring for patients with H7N9 infection, the use of these practices by the attending physicians while caring for the index patient could not be verified. No other common epidemiologic link among these three persons was identified, and all are unrelated.