Novus actus – intervening acts as an element of causation
In the tort of negligence and medical malpractice in particular, i's often easier to establish a breach of duty than it is to establish a causal link between that breach and the injury or loss claimed. That difficulty is compounded where the medical malpractice comes after an event, such as a road accident or injury at work, that has injured a claimant and put them in hospital for treatment that is meant to resolve, cure or mitigate their injury, but makes the situation worse.