Allergy

Tsuranga medtags indicated, among other things, patients' allergies. (TV: The Tsuranga Conundrum)

The Fifth Doctor was allergic to certain gases in the praxis range of the spectrum. As such, he wore a stick of celery in his lapel; if the gas was present, the celery would turn purple and the Doctor would eat it. (TV: The Caves of Androzani)

The Doctor likened the Cybermen's vulnerability to gold to an allergy. (TV: Earthshock)