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.
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.