Spanish influenza
From Tardis Wiki, the free Doctor Who reference
Spanish influenza was a dangerous type of flu that was present on Earth during the 20th century. 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)
