Penicillin

Penicillin was a medicine discovered on Earth by Alexander Fleming in 1928. (AUDIO: The Sirens of Time)

In 1066, the Monk gave some anachronistic penicillin to the injured Eldred, telling Wulnoth that it was a sort of herb. (TV: "A Battle of Wits")

Plummer of the Pharm said that once they'd developed Reset for human use, it would be bigger than penicillin. (TV: Reset)

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 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 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. (AUDIO: The Sirens of Time)