Negative-stained TEM shows recreated 1918 influenza virions that were collected from the supernatant of a 1918-infected Madin-Darby Canine Kidney (MDCK) cell culture, 18 hours after infection. In order to sequester these virions, the MDCK cells were spun down (centrifugation), and the 1918 virus present in the fluid was fixed for negative staining. Dr. Terrence Tumpey, staff microbiologist and member of the National Center for Infectious Diseases (NCID), recreated the 1918 influenza virus in order to identify the characteristics that made this organism such a deadly pathogen. Research efforts such as this enables researchers to develop new vaccines and treatments for future pandemic influenza viruses. The 1918 Spanish flu epidemic was caused by an influenza A (H1N1) virus, killing more than 500,000 people in the US, and up to 50 million worldwide. The possible source was a newly emerged virus from a swine or an avian host of a mutated H1N1 virus. Influenza A (H1N1) viruses still circulate today after being introduced again into the human population in the 1970s. Magnification unknown.