Ohai

it is from a doctor who episode, where an alien ambulance crashes on earth and releases a nano-swarm who go and heal stuff,

BUT they dont know how humans are supposed to look like so they look for one first, and what they find is a dead little boy with a gasmask on that never knew who his mommy was. So every time the nanobots hit something(they get transported by the victims), the nanobots "heal" the victim: place a gasmask on the head, make them look for the boys mommy and have the intelligence of a dead guy