Nanogene
Nanogenes are a type of sub-atomic robot capable of healing superficial wounds in seconds, used as medical equipment by certain advanced races such as the Chula. They can also revive the dead effortlessly for has the Doctor puts it life is nothing to a nanogene just "Nature's way of keeping meat fresh". This means theoretically death can be referced if there are nanogenes present but the nanogenes must have prior knowledge of the species they are going to revive otherwise there can be terrible side effects like the Empty child plague.
Inadvertently, they created the Empty Child plague when they found a dead child wearing a gas mask and inflicted with fatal physical wounds and because they had never seen a human before they believed that this was their default form. Using the child as a template they set about turning all humans to match the attributes of this child. (injuries and gas mask bonded to the face).
The nanogenes that created the plague arrived on earth when the then con man Captain Jack Harkness obtained there burnt out medical unit which he thought was an empty medical transport. He then hijacked the ships controls and vaulted the ship in front of the Tardis and stageing a crash landing hopeing to real in a timeagent he could con saying it was a Chula warship. The landing however did not go has smothly has planed and the ship somehow became damaged, possibly by a bomb has it came down during an air raid, or the local militry damaged it whilst trying to open it. Whatever the case containment was broken and millions of nanogenes; enough to rebuild a species escaped in to the air.
The nanognes then who had been programed with the simple directive to heal any lifeform they came in contact with came across a dead child wearing a gasmask. Now this was where the problem began has unlike the ones of Jack's ship these nanogenes had never seen a human before so they didnt know what a human was supposed to look like, but never the less they do what they where programed to and set about repairing the body not sure whats flesh and whats gas mask but never the less they try there best resulting in the plague being born.
They eventually recognized the real DNA by finding the child's mother, therefore coming to the conclusion that they where incorrect and reversing the process. Since the Chula used these nanogenes in war it also enhanced the subject's physical abilities specifically super-Human strength, telepathic abilities and the ability to communicate using certain mediums (radios, phones and typewriters). (DW: The Empty Child / The Doctor Dances)
Nanogenes were apparently still in use by the year 5 billion, as the Doctor "borrowed" some from the New New York Hospital. (NSA: Peacemaker)
