The RMS Lusitania was an ocean liner torpedoed by the U-20 on 7 May 1915. This act helped provoke the United States into entering World War I two years later. One of the people killed onboard was a petty criminal named Eric Charles Vincent.
In an alternate timeline, the Lusitania did not sink due to the Fifth Doctor's intervention. Vincent, having survived, went on to kill Alexander Fleming in a botched robbery attempt in December 1927 before the biologist discovered penicillin. Without the protection offered by penicillin, Earth fell prey to new strains of meningitis and pneumonia in 1956. The survivors never developed a space programme. Consequently, the Knights of Velyshaa were not defeated by the Earth Alliance in 3562 as they otherwise would have been. (AUDIO: The Sirens of Time)
After Nardole explained the mystery of the Mary Celeste to a group of Picts, he was about to tell them about the Lusitania before the Twelfth Doctor re-emerged from the Gate after two days. (TV: The Eaters of Light)