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"Miracle Day" was the name originally given to 19/20 March 2011, when every human on Earth simply stopped dying. The name, however, still stuck when the phenomenon lasted for months more. It was caused when the Three Families fed the immortal Jack Harkness' blood into the Blessing, which in turn relayed the template to everyone on the planet in seconds.
Without any immediately obvious cause, awareness of the event took time to develop. One of the earliest survivals was Oswald Danes, a convicted prisoner in the United States about to be executed; although his lethal injection was carried out flawlessly on 19 March, the only effect was intense but temporary pain from the drugs. As hours passed, doctors around the planet began to realise that deaths of patients under their care (such as Rex Matheson, who was impaled by a metal pole) had abruptly dropped to zero. Age, disease, trauma, and all other normally fatal conditions continued to hurt and debilitate their victims, but without killing them. The level of damage to the body did not matter. Extreme examples include an attempted suicide bomber whose entire body was almost totally burned and pulverised but retained some degree of consciousness even after his head was detached from the rest of his body (TV: The New World) and a CIA agent whose neck was broken, yet she managed to stand and stumble away, her head facing backward (TV: Rendition). News media and the general public also began to realise the change. Eventually, the name of the event emerged on the back of the word "miracle," which began to trend across social networking sites.
The only thing that seemed to permanently kill humans was complete incineration; so long as the bodies remained intact to a certain degree, consciousness and life was still possible to a point, but otherwise human beings would continue to exist despite the impossibility of death or recovery. Even detached body parts would still be animated. (TV: The New World) However it was later discovered that jumping off a tall enough building caused people to be forever unconscious, yet not dead, allowing suicide to exist to a certain degree. (TV: The Middle Men) Also, under Angelo Colasanto's bed was a null point to the miracle, where mortality was possible again, and death existed in that one place. A null field generator could exclude a person from the effects of the miracle. (TV: End of the Road)
Eventually it was revealed that The Three Families fed Captain Jack Harkness's immortal blood into a mysterious tunnel called the Blessing that controlled Earth's morphic field. This caused it to change the morphic field so that everyone became immortal. The Families had the plan to take over the immortal world using their connections once everything stabilised. Miracle Day finally ended when Jack Harkness and Rex Matheson fed Jack's now mortal blood into the Blessing, causing the Miracle to reverse itself with Jack regaining his immortality and everyone else regaining their mortality (Apart from Rex Matheson, who replaced most of his blood with some of Jack Harkness'). As the Miracle ended, everyone who was supposed to die woke up for a moment, before finally passing away. (TV: The Blood Line)
Without death, the human population began to swell rapidly, increasing pressure on resources and on the species itself. The lack of deaths from any disease and the need to keep treating the sick with new medicines was expected to give viruses and bacteria the chance to evolve into unstoppable "superbugs." It was estimated that within four months, Miracle Day would cause a complete environmental and social collapse. (TV: The New World, Rendition) Beyond medical effects, individuals and societies started to react to Miracle Day in myriad ways:
Doctor Vera Juarez ordered an immediate and radical change to long-established emergency room triage procedures - treating minor injuries before major ones, in order to free hospital beds for the inevitable increase in patients. (TV: Rendition)
The percentage of pregnancies that miscarried due to birth defects no longer did so, while retaining the genetic anomalies, resulting in the potential of babies being born without faces or with exposed brains. (TV: Dead of Night)
People were divided into categories; Category 3 was normal with no life threatening injuries, Category 2 was fatal injuries or illness (but still alive and conscious due to the miracle), and Category 1 was effectively dead, usually deemed as unconscious and unlikely ever to wake up. (TV: The Categories of Life) In order to deal with criminals, a Category 0 was planned, where they would be "executed" like Category 1s. (TV: End of the Road)
Governments were forced to set up "overflow camps" (TV: Escape to LA), in which Category 1 and 2 patients were kept. However, these were, in effect, glorified concentration camps, with Category 1 patients being burned alive. (TV: The Categories of Life)
These are notable cases of people surviving due to Miracle Day along with the condition that would have caused their death.
Oswald Danes survived lethal injection (TV: The New World), from which he had recovered from enough to not die when the miracle was reversed, though he committed suicide to kill the families anyway moments later.
Rex Matheson was impaled with a pole, stabbed with a pen in the same place twice by Colin Maloney, and later replaced most of his blood with that of Jack Harkness. He survived the miracle and (presumably) due to the blood transfusion, became immortal in the same manner as Jack Harkness. (TV: The New World, The Middle Men, The Blood Line)
A suicide bomber was shot in the neck by Captain Jack, at the centre of an explosion, decapitated by a surgeon. Given his catastrophic injuries, he died when the miracle was negated or was sent to the modules (the latter most likely out of mercy). (TV: The New World)
Lyn Peterfield broke neck though later seen with her head rotated 180 degrees on her body, suggesting internal decapitation. She either died in the modules (though when last seen she did not appear to be category one) or when the miracle was negated. (TV: Rendition)
Zheng Yibao jumped off a high building, causing him to be unconscious forever but not "dead". He was a de Facto member of the 45 Club. He died when the miracle was negated or in the modules. (TV: The Middle Men)
Numerous operatives of the Three Families in both Shanghai and Buenos Aires became category ones at the hands of the Torchwood team through shooting or neck-breaking. They either died when the miracle was negated. (TV: The Blood Line)
Jack Harkness, who was formerly immortal, was apparently rendered mortal and vulnerable by the Miracle. He nearly died when he was poisoned (TV: Rendition) and shot (TV: End of the Road) but survived when his immortality was reinstated.
Only humanity was affected by the Miracle, with no other lifeforms on Earth becoming immortal. Had insects been affected, they would have overrun the world within forty-eight hours. (TV: The New World)