Lazarus shtick

The "Lazarus shtick" was John Hart's term for Jack Harkness' immortality. (AUDIO: The Death of Captain Jack)

Origin
After he was first killed when shot by a Dalek, he was resurrected by Rose Tyler as the Bad Wolf. (TV: The Parting of the Ways) While using the power of the Time Vortex to restore Jack, Rose couldn't totally control the power she wielded, and she brought him back forever by accident. As such, he became a singularity in a fixed point of time, an immortal being that the Doctor had trouble even looking at, and even the TARDIS tried to get rid of him. The Doctor said that he was unable to undo Jack's immortality, and Jack was eternal, he would never age past his prime and never truly die. (TV: Utopia)

Nature
Since his first resurrection, whenever he did die, he automatically came back to life almost instantly, each time awaking with a very deep gasp of breath as his respiratory system resumed its functions. (TV: The Parting of the Ways) (TV: Everything Changes onwards) However, on occasion his resurrection was delayed if experienced enough trauma. (TV: End of Days, Children of Earth: Day One) While not understanding the mechanics of his condition, his just-deceased body is apparently held in stasis while time ran backwards over it, repairing wounds and mending the damage. (PROSE: Bay of the Dead) Jack could sense what was done to his body during his brief periods of death, such as when Martha Jones unsuccessfully attempted to resuscitate him using the kiss of life. (TV: Utopia) Jack viewed this power as a curse as much as a blessing, (TV: Everything Changes) Jack describes the process of resurrection as being "hauled over broken glass."(TV: Kiss Kiss, Bang Bang)

Jack could survive heat and radiation which would have burned or vaporised regular humans. (TV: Utopia) He also didn't need an immune system, as his body never needed to build up any resistance, although this meant he would easily fall ill without his immortality. (TV: Rendition) His body could infinitely regenerate, allowing him to automatically restore himself to perfect condition. However severe enough trauma could delay his resurrection, such as suffering the pallor of a corpse before returning to full health. He could even re-grow his whole body; when he was blown up after having a bomb implanted in his stomach, he fully regenerated from just an arm, a shoulder, and part of his head in a bit over twelve hours. His regeneration process began with growing bones, followed by his internal organs, and lastly his flesh and skin. He also retained all his memories and knowledge as well.

The process of resurrection could often be very painful, especially in this instance. He regained consciousness before his healing was complete, with his eyes and his flesh yet to regrow, but the ability to feel pain intact, causing Jack to scream for hours on end and hold extreme resentment to those who planted the bomb inside him. (TV: Children of Earth: Day Two) Pain appeared to be dependent on the amount of damage to the body, with more pain in cases where foreign objects remained within the wound. (TV: Fragments, Children of Earth: Day Two) Asphyxiation in an airless environment would make him die and instantly revive in cycle endlessly if he was never unearthed. (TV: Exit Wounds) When asked by Ianto what would happen if his body were totally destroyed, Jack merely said "Maybe we ought to try it some time". (PROSE: Bay of the Dead) Notably, although Jack quickly recovered from fatal injuries, minor injuries sustained like a cut lip or a black eye would remain and heal at the rate of a normal human. (TV: Cyberwoman, Fragments)

Deaths
In 200,101, Jack was forced to interact with live cables to safely land Trear Station on Earth, electrocuting him. Reviving, Jack reasoned that the universe "like[d]" him. (AUDIO: The Year After I Died)

Using a vortex manipulator to travel to Earth, ending up in 1868, Jack did not realise he was immortal until 1892, when he woke up after a man shot him through the heart in Ellis Island. Over a period of at least a century Jack waited for the Doctor. Before reuniting with the TARDIS in 2008, Jack had died by falling off a cliff, being trampled by horses, World War I, World War II, poison, starvation and a stray javelin. (TV: Utopia)

Shortly after meeting Gwen Cooper, Jack was shot in the head by Suzie Costello. In a matter of seconds, Jack revived with his head wound healing. (TV: Everything Changes) Confronting the partially cyber-converted Lisa Hallett, Jack was, in quick succession, fatally electrocuted by her twice, noting that he was not so easily "deleted". (TV: Cyberwoman)

While attempting to stop his teammates from using the Rift Manipulator, Jack was shot in the head by Owen Harper. Though he revived soon after, Jack was still physically weakened as he faced Abaddon, a giant demon who fed on life that was rampaging through Cardiff, killing all in his shadow. Jack, noting that he would be an "all you can eat buffet" for Abaddon, stepped into his shadow. In what proved to be a painful process for Jack, his excess life energy eventually killed the demon before Jack perished as well. His body was taken back to the Hub, where Owen found no vital signs, deeming him "ice cold". However, after a few days, Jack revived once again. (TV: End of Days)

Aboard the Valiant, Jack was shot dead by the laser screwdriver wielded by who, aware that Jack was immortal, gleefully noted that he could kill him again. (TV: The Sound of Drums) Throughout the Year That Never Was, Jack was held prisoner aboard the Valiant. By the end of the year, Jack made an escape attempt which resulted in him being shot dead by the Valiant's security personnel. (TV: Last of the Time Lords)

Shortly following the Year That Never Was, Jack reunited with Captain John Hart, who threw him off a building to his death. It was only after Jack returned to the Hub that John realised he was now immortal. (TV: Kiss Kiss, Bang Bang) Later, Jack was killed by Death. (TV: Dead Man Walking)

Between his first encounter with Torchwood and his reunion with Gray, Jack had died a total of 1,392 times. (TV: Fragments) In the Hub, Jack was fatally shot by John Hart, who restrained him before he could revive. John, who had been coerced by Gray, took Jack back to 27 AD, where he was buried alive in the soil of Cardiff, as his brother made his intentions clear. (TV: Exit Wounds) "I want you to suffer. I want your life. This is Cardiff, 27 AD. The city will be built here, over the next two thousand years. Your grave will be the city's foundations. Your blessing of life becomes a curse. Each time you revive, with a throat full of earth, each time it chokes you afresh, and you thrash on the edge of death, you think of me."

- Gray.

Eventually, Jack was uncovered by Torchwood Cardiff members Alice Guppy and Charles Gaskell in 1901, at a point in time where his past self was working for them. Avoiding interacting with his earlier self, Jack instructed them to freeze him in the morgue for 107 years. (TV: Exit Wounds)

During the 21st century Dalek invasion, Jack was exterminated by the Supreme Dalek aboard the Crucible, discretely reviving in seconds. (TV: The Stolen Earth)

At St Helen's Hospital, Jack was shot dead by Rupesh Patanjali on behalf of the British government, whom intended to destroy Torchwood. Reviving, Jack returned to the Hub, unaware that a bomb had been planted within him. By chance, Jack discovered this as the bomb was exposed by the scanner which confirmed Gwen's pregnancy. With a blast radius of one mile and only two minutes before detonation, Jack remained behind as he ordered Gwen and Ianto Jones to leave before the bomb went off, blowing Jack's body apart and destroying the Hub. (TV: Children of Earth: Day One)

During the Gallipoli campaign, Jack was shot dead by Ata, then again by Ottoman forces when they recovered Ata. (AUDIO: What Have I Done?)

In an alternate timeline created by John Hart, he used distilled rift energy with technology from the Torchwood Institute to cut Jack off from the Time Vortex, nullifying his immortality and transfering it to himself. Jack, however, remained a fixed point in time and, after 130 years, died of old age which resulted in John's universe collapsing in on itself. (AUDIO: The Death of Captain Jack)

Other references
Catching up with the Ashildr, who had been made immortal by a Mire repair kit, in 1651, the Twelfth Doctor acknowledged Jack as another immortal and suggested that she would meet him eventually. (TV: The Woman Who Lived)

During the 2021 Security Drone Incident, Jack boasted to the Death Squad Daleks that he was immortal before he safely escaped the destruction of their spacecraft. (TV: Revolution of the Daleks)