What makes Marvel's Deadpool so annoying to his enemies (and allies) is that he isn't just hard to get rid of: he's effectively unkillable. Thanks to Deadpool's healing factor he can regenerate from almost any injury, including being literally torn in half or decapitated. His extreme resilience to damage has led many people to speculate that he may in fact be completely immortal.
Immortality is nothing to scoff at, even in the world of comic books where characters treat the afterlife like a waiting room before they're resurrected. Deadpool's immortality comes from his extreme healing factor, which constantly replaces his damaged cells with healthy ones at such an absurd rate that he's actually immune to telepathy because his brain cells are being repaired faster than any telepath can mess with his mind. This is just one example of how extreme Deadpool's regeneration powers are, but is his healing factor really enough to make the Merc with a Mouth live forever?
Deadpool Got Wolverine's Healing Factor (With a Twist)
Deadpool is, contrary to appearances, not a real mutant. His superpowers are the result of being experimented on by the Weapon X program, the same program that gave Wolverine his healing factor. But the difference is that while Wolverine was already a mutant before the experiments, Deadpool was... just some guy with cancer. Weapon X had replaced Wolverine's normal bones with an Adamantium skeleton, and it transplanted his healing factor onto one Wade Wilson, who would then go on to become the wise-cracking, fourth-wall breaking Deadpool we all know and tolerate today. So since Wolverine is effectively immortal (and has been shown to still be alive countless centuries into the future of the Marvel Universe) it would follow that Deadpool, who has Wolverine's powers, is also immortal to the same degree as Logan. Case closed, right? Wrong.
Wolverine Will Die, But Deadpool Won't
The problem with this theory is that Wolverine actually has been permanently killed before. Yes he did come back to life eventually, but only because of outside help unrelated to his healing factor. Not only that but it's been established that while Wolverine won't die right now his healing factor does slow down over time and he will eventually grow old and die. This fact is the entire conceit behind the 2017 film Logan as well as the comic storyline that the movie was based on, the appropriately titled Old Man Logan story arc written by Mark Millar in 2008. Not only that but it's also been established that even with his healing factor, Wolverine is still slowly dying due to the his his adamantium skeleton poisoning him.
Deadpool actually has a similar problem to Wolverine. Because he was given a healing factor while he had cancer, Deadpool's healing doesn't just restore his normal cells but his cancer cells as well. This means that Deadpool will be stuck with cancer for all eternity. Deadpool's healing factor on the other hand has never been shown to have any limits or slow down. So for whatever reason, his regeneration is actually stronger than the man who gave it to him.
Yes, Deadpool is Immortal in Marvel's Distant Future
In the recently published 2020 comic Deadpool: The End readers are shown a flash-forward to the future of the Marvel Universe where almost every other superhero had already died-except for Deadpool. This is just the most recent in a long tradition of comics hat show alternate futures where Deadpool is still alive and kicking despite most other "immortal" heroes dying. What is is about Deadpool specifically that lets him avoid death? The answer lies with his romantic rival and cosmic jewel collector, Thanos.
Thanos and Deadpool seemingly have nothing in common at first but they're actually part of a superhero love triangle with Marvel's personification of death. Thanos loves death, but Death loves Deadpool. Deadpool also loves Death, so Thanos decided to curse Deadpool so that he will never die to keep the two apart. This helps explain why Deadpool's immortality is so much better than other characters with similar powers.
Of course, comic books aren't always consistent. And there have been other alternate futures where Deadpool is missing and presumed dead. In Jason Aaron's Thor #1 the only two people left alive right before the heat death of the Universe are an elderly Thor and Wolverine with phoenix powers. Similarly Mark Waid's History of the Marvel Universe has Franklin Richards and Galactus as the last survivors of universe-616. But these are just alternate futures with the potential to happen-in the present, Deadpool has shown himself time and again to be impossible to kill. Ultimately it's up to the writers to decide what, if anything, will finally be able to kill Deadpool once and for all.