Spanish influenza

Spanish influenza was a dangerous type of flu that was present on Earth from 1918-1919 and was the most sever 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 12,500,000 deaths occurred. The United States of America suffered approximately 550,000 deaths and a total estimated 30 million people died throughout all populated areas of the world. (EDA: Casualties of War)

In 1965, a new strain of the this flu, which could kill millions, was discovered. The 456 contacted Earth and offered an anti-virus, in exchange for twelve children. Jack Harkness took up their offer. (TW: Children of Earth: Day One, Day Three, Day Four)