In Series 9, "The Girl Who Died", we're introduced to Ashildr, the viking girl whose life the Doctor saved by using a Mire "Medical Kit", which will keep "repairing" her, maybe forever.
The Doctor: It won't stop, the repair kit I put inside Ashildr, not ever. It'll just keep fixing her.
Clara: Well, good.
The Doctor: I'm not sure, but it's entirely possible she has lost the ability to die.
Clara: The ability?
The Doctor: Oh, dying is an ability, believe me. Barring accidents, she may now be functionally immortal.
As the series went on, we discovered that Ashildr
effectively lived forever. Right to the end of the universe, which is still 100 trillion years away, according to some theories. The Mire medical kit kept curing her of everything, and healing every injury, not to mention that she even stopped aging.
So, why was she the only immortal? If those medical kits are used by Mire warriors, why aren't they immortal as well? Did the Doctor tamper with it? Does it only make humans, or humanoid species immortal?