An outbreak of avian H7N9 influenza in humans was reported in early 2013. Structural and infection studies are helping to reveal how these viruses can adapt to infect, and potentially transmit in, new species. See Letters p.496 & p.500 The H7N9 avian flu virus emerged in the human population on mainland China in February 2013, and by the first week of July WHO had recorded 133 cases including 43 deaths. Most cases so far have been linked to live bird markets. In this issue of Nature two groups report on the receptor-binding properties of H7N9. Both find that the virus has acquired the ability to bind the human α-2,3-linked sialic acid receptor yet has a retained preference for binding to the avian 2,3-linked receptor, a factor that may restrict its further evolution towards efficient transmission between humans. Steven Gamblin and colleagues also solve the crystal structure of the H7 haemagglutinin in complex with the receptor analogues, revealing details of how the human-receptor-binding properties may have arisen. Yuelong Shu and colleagues examine the pattern of virus infection in lung tissue. In human tracheal and lung explants, the virus infects epithelial cells in the lower respiratory tract and type II pneumocytes in the alveoli, and is better able to replicate in the lower respiratory tract compared with the trachea, a possible factor in the inefficient human-to-human transmission seen to date. They also report hypercytokinaemia in some patients — a cytokine storm that can contribute to disease severity — comparable to that seen in some H5N1 infections.