Certain interpretations of Quantum Mechanics suggest that all possible ‘choices’ of events occur in a near-infinite array of parallel worlds. Given that, at each point in time, in some possible worlds you die for whatever reason, in others you don’t.

Whatever event occurs, there’s some slim chance of your survival (even if that slim chance relies on some freaky quantum miracle, like all the atoms in your body simultaneously leaping a metre to the left, out of the path of a bullet) so there will always be some possible world in which you survive whatever it is that’s trying to kill you.

You can only experience a world in which you are alive, therefore any time you do something potentially lethal your experience must, necessarily, be of a world where you survive. Therefore everyone is immortal (they just aren’t all immortal in the same universe at once… except in that one, distantly improbably universe where everyone cheats death every time).

[The above is probably bull****]