Spanish influenza

Spanish influenza was a dangerous type of flu that was pandemic on Earth from 1918-1919 and was the most severe outbreak of the 20th century. After some time, the virus mutated into a more lethal strain and a second, more severe form emerged in August 1918. In India at least twelve and a half million deaths occurred. The United States of America suffered approximately five hundred fifty thousand deaths and a total estimated thirty million people died throughout the world. (EDA: Casualties of War)

In 1965, Venessa compared a strain of Indonesian flu she was negotiating an antivirus for to this outbreak. (TW: Children of Earth: Day Four)

In 2007, Wilfred Mott was unable to attend his granddaughter's aborted wedding to Lance Bennett after falling ill with a non-lethal strain of Spanish flu. (DW: The Runaway Bride, The Sontaran Stratagem)