RMS Lusitania

The RMS Lusitania was an ocean liner that was torpedoed by a German U-boat on 7 May 1915, an act that 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 alternative timeline, the Lusitania did not sink due to the Doctor's intervention. Vincent, having survived, went on to kill Alexander Fleming in a botched robbery attempt before the biologist discovered penicillin. Without the protection offered by penicillin, Earth fell prey to meningitis and pneumonia in 1956. The survivors never developed a space programme. (BFA: The Sirens of Time)