The morbidity and economic tolls of influenza virus are huge, regardless of its capacity to kill. Vaccines and therapies to control this persistent threat are limited. In structural studies, Kadam and Wilson show how the broad-spectrum antiviral arbidol inactivates viral hemagglutinin (HA). HA is a surface glycoprotein that recognizes the host and mediates virus fusion and disgorgement of nucleic acids into the cell. Arbidol binds in hydrophobic cavities in the upper region of the HA stem, creating a network of interactions that makes the molecule rigid and prevents cell fusion. Resolving the molecular details of the arbidol-HA interactions is essential for the optimization and global deployment of this potential new influenza drug.