Meningitis

Meningitis was a potentially fatal disease.

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. (BFA: The Sirens of Time)

In Peri Brown's dream on YT45, her son Kyle was diagnosed with meningitis. (BFA: The Mind's Eye)