Pneumonia

 was a potentially fatal disease to at least humans. Sir Francis Bacon and Ian Stratford's mother were known to have both died from it. (PDA: Heart of TARDIS, EDA: 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. (TVA: Steelfist)