H1N1 1918 influenza virus particles (brown), coloured transmission electron micrograph (TEM). These virus particles (virions) have been recreated from the influenza (flu) virus strain that caused the 1918 Spanish flu pandemic. This pandemic infected a fifth of the world population and killed between 20 and 50 million. It was reconstructed by scientists at the Center for Disease Control and Prevention (CDC), Atlanta, USA, in 2005. Scientists hope to identify the traits that made the virus so deadly and, in doing so, develop new vaccines and treatments for future pandemic influenza viruses. These virions were collected from an MDCK culture cell 18 hours after infection. The 1918 virus was an H1N1 form of influenza A.