Quantum Immortality is the theory that, no matter what you do, you will live indefinitely assuming that there are an infinite parallel universes. For example, you could be in the electric chair, and you will almost definitely become electrocuted; but, in a parallel Universe, under investigation of all possible consequences of pulling the chair’s trigger, the chair will cease to work — leaving you to be conscious in another parallel universe without any recollection of your death in your “original” Universe.
You could have died infinitely, in an infinite amount of possible ways, already, but you keep switching from Universe to Universe — leaving you, essentially, immortal. Of course, you could argue that perhaps your consciousness will eventually pick up on accidents – but the case cannot be true, since only the potential for any possibility can leave you to survive death, and any explanation for your survival, such as the electric chair’s malfunction, can be explained rationally by past events that took place in that Parallel Universe.
There is the case that you could purposefully attempt Russian Roulette. In one Universe, you may have died from the game; yet, after gradual attempts, including buying a new Revolver to replace the possibility of malfunction, you could die. But what if you regained consciousness in a Universe where a series of events could have outright banned the production, and possession, or any guns, while leaving you with a recollection of your new Parallel being’s consciousness? Or, what if a man were to grab your Revolver and stop you from doing it in another Parallel Universe? Perhaps, provided there is such thing as using medicine to keep oneself alive indefinitely, you would theoretically live forever — since an infinite amount of Parallel Universes, all differing by a Butterfly effect, leads to an infinite amount of possibilities for life.
One should be cautious when considering the Quantum Immortality theory so as not to delve into a Quantum “woo”. The quantum woo is often perpetrated by New Age spiritualists with a lack of scientific understand, where they talk about ridiculous claims (i.e. electric brain waves, chakras, astral projection etc.) and then attempt to justify the claims by expressing a profound lack of understanding about quantum physics.