H1N1 flu virus particles (virions, orange) on a cell (green), coloured scanning electron micrograph (SEM). This is the Neurotrophic Wilson-Smith (NWS) strain of an influenza A virus subtype H1N1. Influenza A can infect multiple species and is the most virulent human pathogen of the three influenza types (A, B and C), meaning that it has the potential to cause a pandemic. Magnification: x15,000 when printed 10 centimetres wide.
Credit: NIBSC/SCIENCE PHOTO LIBRARY