Haemagglutinin protein, 1918 flu virus. Molecular model of a surface protein (haemagglutinin) from the surface of the deadly influenza A virus that caused the pandemic of 'Spanish flu' in 1918. Haemagglutinin binds the virus to the cell membrane before it enters the cell. The haemagglutinin on the 1918 virus was particularly efficient at binding to human cells, partly accounting for the virulence of the outbreak, estimated to have killed 50-100 million people worldwide. Here, it is being studied with the avian H1N1 virus.