When conman Captain Jack Harkness planted a Chula ambulance in 1941 London, the Chula nanogenes, who had never previously encountered humans, infected a dying human boy named Jamie caught in the London Blitz. Assuming that the gas mask he was wearing was part of his face, they "healed" him, and unintentionally created the "Empty Child," a "zombie" based on Jamie's characteristics and injuries at death. Thus, everyone affected by the plague inherited a gas mask in place of their faces, severe head trauma on the left side, partial collapse of the chest cavity of the left, and a gash on the back of their right hands.
The nanogenes also gave the child the standard abilities of Chula soldiers, specifically super-human strength, telepathic abilities and the ability to communicate using anything with a speaker grill. The Empty Child still had a child-like mentality and escaped to search for its mother.
The Empty Child compulsively searched for his mother, and, in doing so, contaminated other humans with the nanogenes. Those who were infected did likewise so that the nanogenes spread in a fashion similar to a virus. The child, and all victims of it, were infamous for repeating the phrase, "Are you my Mummy?", endlessly.
The Empty Child plague was finally stopped when the Ninth Doctor reunited the Empty Child with his mother and the nanogenes understood, recognising the mother as the superior source, the proper sequence for a human of that era. (TV: The Doctor Dances)
The Tenth Doctor, after having assumed the identity of John Smith, drew a picture of two Empty Child victims in A Journal of Impossible Things. (TV: Human Nature) When later given a gas mask during Sontaran invasion of Earth, the Tenth Doctor jokingly asked, "Are you my mummy?," as a reference to these events. (TV: The Poison Sky) When confronted with the Foretold, a mummy on the Orient Express in space, the Twelfth Doctor attempting to find out how and why the mummy was present, blurted out, "Are you my mummy?". (TV: Mummy on the Orient Express)