This image shows part of the 1918 Spanish flu virus in complex with an antibody that also neutralizes the 2009 "swine flu" virus. The antibody 2D1 Fab is depicted as red and yellow ribbons. The hemagglutinin on the flu virus is depicted as a solid surface. One of the three hemagglutinin subunits is highlighted in blue and cyan. The Sa site—the region conserved between the 1918 and swine flu viruses and targeted by the 2D1 antibody—is highlighted in magenta. (Image courtesy of the Scripps Research Wilson lab.)