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. Inadvertantly, they created the Empty Child plague when they found a dead child with a gas mask wounds they set about turning all Humans like this as they belived this was the Human form. They eventually recognised the real DNA and reversed this. (DW: The Empty Child, The Doctor Dances)