Cryo-EM image of the Influenza A (H1N1) virus, PR8 strain. Influenza A (H1N1) virus is the subtype of influenza A virus that was the most common cause of human influenza (flu), and is associated with the 1918 outbreak known as the Spanish Flu. In 2009, the World Health Organization (WHO) declared the new strain of swine-origin H1N1 as a pandemic. This strain is often called swine flu by the public media. This novel virus spread worldwide and had caused about 17,000 deaths by the start of 2010. In 2010, WHO declared that the H1N1 influenza pandemic over, saying worldwide flu activity had returned to typical seasonal patterns. Cryo-electron microscopy (cryo-EM), or electron cryomicroscopy, is a form of transmission electron microscopy (TEM) where the sample is studied at cryogenic temperatures (generally liquid-nitrogen temperatures). Cryo-EM is popular in structural biology. Magnification unknown.