An injury was damage to the body caused by external force. 

An injury could be the cause of regeneration. (TVLogopolisTime and the RaniDoctor WhoUtopiaLet's Kill HitlerHell BentThe Doctor FallsTwice Upon a TimePROSEInterference - Book Two

When the Twelfth Doctor broke his toe, Clara Oswald suggested that he regenerate to heal the injury, but he berated the idea as a waste. (PROSEThe Blood Cell)

When Chula nanogenes found a dying child named Jamie in the London Blitz, they took his injuries as a template for all humans. When they went on to spread the Empty Child virus, those same injuries — severe head trauma on the left side, partial collapse of the chest cavity of the right, and a gash on the back of the right hand — presented themselves in everyone who caught it. (TV: The Empty Child / The Doctor Dances)

Humans also had an innate ability to heal from injuries, on a far smaller scale. (AUDIO: The Invention of Death)

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