Spanish flu virus. Transmission electron micrograph (TEM) of 1918 H1N1 influenza (Spanish flu) virus particles (virions, round/oval) near an infected cell (bottom right). The 1918 influenza (flu) pandemic (January 1918 to December 1920), known as Spanish flu, was an unusually deadly influenza pandemic and the first of two pandemics involving the H1N1 influenza virus. It infected 500 million people worldwide and resulted in the deaths of 50 to 100 million (three to five percent of the world's population), making it one of the deadliest natural disasters in human history.