In ‘The Empty Child/The Doctor Dances’ the Doctor learns that the release of Chula nanogenes have mistakenly used the mangled, body of a boy wearing a gas mask as a template for reconstruction. For a moment he doesn’t see a way out of the situation.
He envisions time being rewritten, the whole world remade in this twisted image by machines that don’t know any better. If not for the discovery that Nancy was the boy’s mother and the nanogenes recognising her DNA to allow them to undo the damage they’ve caused that very well might have happened.
Firstly this is an good example of how the most benign technology can wreck havoc on other worlds. The more advanced and self-sufficient technology becomes the less guidance and control there is.
In this situation the nanogenes encountered a race that it was unfamiliar with but was driven to try to fix it anyway. This is similar to the situation in ‘The Girl Who Waited’, where Amy is nearly killed by medical robots unfamiliar with her species.
In particular nanobots have the capacity to do terrible damage since they can alter, assemble and disassemble on a molecular level. Eric Drexler in his book ‘Engines of Creation’ posits a grey goo end of the world scenario in which nanobots convert all matter into more nanobots, replicating exponentially. In effect reducing everything to grey goo at an incredible and horrifying rate.
We can see the gasmask zombies as a larger scale version of this. Every human being encountered would be converted, who in turn could convert others. Even the dead could be brought back to life in this new form to join their ranks.
The restriction on this is that the gasmask zombies aren’t driven to seek out and convert others. Rather they share the same instinct as Jamie (the template for their design) to find his mother.
Since they spend at least some of their time inert this could reduce their expansion greatly. Still, without a solution the infection would spread, particularly if Jamie decides to expand his search radius.
If events had gone differently and Nancy hadn’t provided a solution what would the Doctor’s next step have been? Containment seems the most obvious solution but it is doubtful he’d have the resources (unless he expanded the TARDIS to encompass the whole of London).
The 9th Doctor may have run, bundling Rose and Jack into the TARDIS and escaping. History would have been rewritten in their wake but maybe the Doctor would hope he could think of a solution later. This could lead to an interesting story arc, with the Doctor adrift in a new timeline, seeking allies and resources while Rose and Jack cope with the loss of everything they now.
In the Doctor’s absence how would time have changed?
It is unfortunate that the outbreak occurred during such a crucial moment in history. With World War II still raging it is doubtful that the government would be able to do anything to contain the infection, especially with bombing raids undoing any quarantine efforts.
Slowly but surely the gasmask zombies would spread out, but it would probably be months before it reached all regions. During this period survivors could make efforts to hide and set up defences.
During this period the British war effort would likely collapse, leaving the country vulnerable to German invasion. Invading troops might mistake the gasmask zombies for a British secret weapon at first.
Before long German soldiers would be infected and probably rushed back to France to either find a cure or be subjected to examination. This would allow the infection to spread throughout Europe.
This could cause the war to continue on for decades, each side trying to take advantage of the infection before they too are infected. It is possible that America, Australia and isolated islands could shut their borders, either preventing its infection or slowing its expansion.
By the 1980s the majority of the world’s population could have been converted. Society would crumble, allowing exploration of various post-apocalyptic settings. Given Jamie’s ability to speak and listen through any electronic device it is likely that such technology would have fallen out of favour, further reducing living conditions.
The early 21st century might not have any uninfected humans left. The whole of mankind could be shambling, immortal gasmask zombies endlessly looking for their mummy. With everyone under his control but still not having found his mother would Jamie look elsewhere, like the stars?
Various alien invaders might initially view Earth as a soft target, until they find that they can’t kill the gasmask zombies. While they don’t have to worry about being converted (unless their DNA is close to that of a human) they might still have their ships and technology taken from them.
Is it possible, with enough time, that Jaime and the gasmask zombies could work out how to use these space vessels to explore the universe to look for his mummy? Early attempts would probably lead to disaster but there is a slim chance they could spend hundreds (if not thousands) of years searching world after world.
PCs might encounter this timeline, forcing them to survive and work out where the divergence happened. They could team up with 51st century Time Agents who have lost contact with their own time period and are desperately trying to fix the mistake of one of their rogue agents.
They could encounter a similar situation on other planets. There are many different causes for zombie-like infections. With a universe teeming with life there are plenty of victims waiting to be converted.
This can give an adventure or even a campaign a strong horror feel. The PCs must not only avoid infection but defend themselves from those who are infected, find a way to contain it and eventually cure it all while preventing others from being converted as well.
Add time travel and things become even trickier. What if the Doctor was only able to get Rose and Captain Jack into the TARDIS after they’d been infected and unknowingly spread it to other worlds?