Nanogenes were a type of sub-atomic robot capable of healing wounds and restoring lost appendages. They could also act as a defence mechanism, creating soldiers capable of protecting or guarding planets and technology.
Nanogenes looked like minuscule, glowing specks that buzzed around their targets. They were capable of healing superficial wounds in seconds and even 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. If a patient had lost body parts such as a limb, the nanogenes could regrow a new one from scratch.
However, upon identifying the first member of an uncatalogued species nanogenes could confuse their biology as part of the overall template, including wounds and body equipment, and then reconfigure that person's body in a misshapen form and any others who came in contact with the nanogenes. In this respect, nanogenes were capable of being guided or outright reprogrammed to identify their mistakes and assume a correct template. Nanongenes could also be deactivated altogether to prevent accidental transformations and misuse of the nanogenes, as well as hide their presence on worlds where the technology was introduced before its time.
The Chula used nanogenes in war. These nanogenes enhanced the subject's physical abilities, giving them superhuman strength, telepathic abilities and Om-Com capabilities. They also could reside 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. (TV: The Empty Child/The Doctor Dances)
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 time the ship was damaged and millions of nanogenes released. This led to the Empty Child plague when the nanogenes found a dead child, wearing a gas mask and with fatal wounds. As they had never encountered a human before, they believed that this was their default form. With the child as a template, they set about transforming all humans to match his attributes, 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 reversed the process. In addition to this, they also restored the missing leg of Mrs Harcourt and healed a minor burn the Ninth Doctor suffered on the palm of his hand. (TV: The Empty Child/The Doctor Dances)