Nanogene

The Nanogenes were a type of sub-atomic robots capable of healing superficial wounds in seconds and even reanimate the dead, provided they had a existing template for the individual. They were used as medical equipment by certain advanced races such as the Chula. Since the Chula used these nanogenes in war, it also enhanced the subject's physical abilities, giving them super-Human strength, telepathic abilities and the ability to communicate using certain mediums (radios, phones and typewriters). They where also capable of residing within a human host like the Empty child and would emrge visably when something of intrest was reavealed. Such has when they encontered Jaimie's mother Nancy and realised there error after scanning her when they recognised her has Jamie's mother and corrected the alterations they had performed on him. Visably they look like glowing flying orbs.

History
In 1941, the con-man Captain Jack Harkness crashed a Chula ambulance in London, planning to sell it and then allow it to be destroyed by bombing. At some point, the ship was damaged and millions of nanogenes were let out. Inadvertently, created the Empty Child plague when they found a dead child wearing a gas mask and inflicted with fatal physical wounds. As they had never encountered 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, with injuries and a gas mask bonded to the face.

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. (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)