Nanogene

The Nanogenes were a type of sub-atomic robots capable of healing superficial wounds in seconds and even of reanimating the dead, provided they had an 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 superhuman strength, telepathic abilities and the ability to communicate using certain media (radios, telephones and typewriters). They were also capable of residing within a human host such as the Empty Child, and would emerge visibly when something of interest was revealed, such as when they encountered Jamie's mother Nancy and realised their error after scanning her; they recognised her as Jamie's mother and corrected the alterations they had performed on him. Visibly 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. This led to the Empty Child plague when the Nanogenes 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 transforming all Humans to match the attributes of this child, with injuries and a gas mask bonded to the face.

They eventually recognised the real DNA by finding the child's mother, therefore coming to the conclusion that they were 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)