RMS Lusitania

The RMS Lusitania was an ocean liner that was torpedoed by the U-20 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 Fifth 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)