There's an old maxim in personal injury law that a defendant takes a plaintiff as he finds him. Stated otherwise, a defendant is responsible for the harm done, regardless of the surrounding circumstances.
Let's say you are involved in an auto accident and injure your back. Even if you have a history of back injuries, a defendant will still be responsible for the additional harm done. One way a skilled plaintiff's attorney will "hammer this point home" to a jury is through a jury instruction should the Judge allow it.
If presented properly, a judge will instruct the jury that a defendant is liable if his negligent action "aggravated" (that is, made worse) a prior condition or injury. This can be particularly useful in situations where the defendant attempts to argue that even if he was negligent -- at fault for the injury in the given situation -- his actions didn't cause the particular injury. A jury instruction of "aggravation" will refute this argument, and it carries a certain aura of authenticity with it.
Essentially, a defendant can certainly argue that his actions didn't cause the injury, but a jury instruction of "aggravation" will be right in front of the jurors as they deliberate, reminding them that even if they find that the defendant took an existing condition and simply made it worse, the defendant should still be held accountable for this additional damage done.