The Empty Child "plague"[source needed] was a technological plague which spread around a part of London in 1941. The Ninth Doctor and Rose Tyler encountered the Empty Child and helped play a part in its cure.
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 right side of their chest cavity, 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. In doing so, it contaminated other humans with the nanogenes. When Nancy was chained to a chair next to an infected soldier called Jenkins, she explained the process of the infection to him as she had seen it happen before. It started with the gash slowly appearing on the back of the infected's hand. Then the victim would incorporate the word "mummy" into their sentences by pure unaware instinct as Jenkins had done to Algy before he left the tent. Then, the victim would lose all mental awareness of who they were, like when Jenkins started to forget things about his life when Nancy questioned him. Then, the victim would feel as if something was "forcing its way up [their] throat". Then the physical transformation would begin. Afterwards, the now transformed victim would remain docile until called into action.
Those who were infected by the virus infected others likewise to how the Child had done 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?". At first, people were only infected if touched by someone who was already infected, though the plague soon became airborne. If not resolved, the Doctor calculated that the virus could convert the whole of humanity in mere hours.
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. Jamie was fully restored by the nanogenes and the Doctor sent the reprogrammed nanogenes to the other infected humans. Now knowing what a proper human was supposed to look like, the nanogenes restored the people back to their original selves and repaired all of their injuries and ailments in the process. The Doctor also programmed the nanogenes to shut themselves down once they repaired the damage they created to stop them from being a problem again in the future. (TV: The Empty Child, The Doctor Dances)
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)
Behind the scenes
Victims of the Empty Child "plague" under the name "Gas Mask Zombies" are used as enemies in the Doctor Who: Legacy mobile game.