The prosecution must prove factual and legal causation when deciding if the D is guilty or not - the thin skull rule may also apply.
Factual causation requires that the consequence would not have occurred 'but for' the D's act or omission; it uses the 'but for test'.
In Paggett, the D used his girlfriend as a human shield when he shot at the police; when they retaliated, the shot her and she died. Even though he did not personally shoot her, she would not have died but for the D's actions.
The D's actions/omissions must be a substantial and operative cause of the consequence in order for them to be guilty. The D's actions/omissions do not have to be the only cause of the consequence, but they need to have significantly contributed to it. Here, the 'significant contribution test' is applied. This was said in Paggett by Lord Goff.
In Smith, the D stabbed the V in his lung and on the way to the hospital he was dropped twice and then at the medical centre, he was given artificial respiration by pressing on his chest. When the V died, there were a few intervening acts, but the D's act was a substantial and operative cause of the V's death.