The Fourth Doctor finds Sarah's arm to be "solid as a rock", supposedly from tetanus. (TV: The Hand of Fear)
Tetanus was a disease. Sarah Jane Smith was given anti-tetanus treatment, after being hospitalised, due to muscular contraction in her hand and forearm. An intern theorised that, alternatively, her symptoms could be a reaction to stress. (TV: The Hand of Fear)
The Fourth Doctor later mentioned you could catch all sorts of things off a dirty knife, including tetanus. (TV: The Stones of Blood)
