Alexander Fleming

Alexander Fleming was a human biologist who discovered penicillin.

In an alternative timeline accidentally created by the Fifth Doctor, the RMS Lusitania did not sink on 7 May 1915 and a petty criminal named Eric Charles Vincent (who died in the sinking of the Lusitania in the proper timeline) killed 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 and 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. (BFA: The Sirens of Time)