Pneumonia

 was a potentially fatal disease of the lungs in, at least, humans. Sir Francis Bacon and Ian Stratford's mother were known to have both died from it. (PROSE: Heart of TARDIS, PROSE: The Banquo Legacy)

The Third Doctor was also susceptible to it — though it was not known if was fatal to Time Lords or if it even had the capacity to trigger regeneration. When once he emerged from the Atlantic Ocean after "General Steelfist" had him driven off a British cliff, the Doctor complained that he was going to catch pneumonia if he didn't get into dry clothes. (COMIC: Steelfist)

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 pneumonia and meningitis 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)