The planet itself was slightly inspired by a comic strip that ran in Doctor Who Adventures a few weeks ago, but obviously I don’t want to just copy their ideas, so I had to come up with some kind of story involving The Doctor finding a man in a thoroughly inhospitable place and following it up because he is a nosy parker and can’t leave anything alone.

Ty Vien, actually came from a bit of chatroom dialogue that I noticed on my partner’s computer – which is next to the coffee percolator. Ty, in chatroom speak means Thank You, and Vien was somebody’s chat nickname. But it struck me as a good name for a planet or an alien community. So Ty became the surname of the people in the ‘Vien’ which became Alterian for ‘tribe’ or ‘community.’

Having put their Vien underground, I then actually started to think about Watership Down. There is a warren in that story which is overcrowded, where the rabbits have to go up to eat and exercise in shifts, and where any rabbit who looks remotely ill is thrown out to die. The analogy with the Vien was perfect. Here we have a people who are overcrowded, have a strict rota for use of their facilities and are paranoid about illness.

I gave a certain amount of thought to what the disease would be that was sweeping through the Vien. Obviously just about anything would take hold easily in such a community. But I didn’t want to be something really dangerous like Ebola or Smallpox, or even influenza, which would be deadly in these circumstances.

Rubella is a relatively easy disease. It is dangerous only to the very young, the very old, and pregnant women – or more specifically the unborn children. It is certainly not a plague. But a society like the Ty Vien, who had lost touch with the outside world and who had obviously lost a lot of medical knowledge, would be bound to overreact.

It is significant that Stella recognised the disease slightly before The Doctor. It represents how much Stella has grown in her time in the TARDIS. This is her penultimate story, so I wanted to show that she really has come on from the girl whose mind was full of boys and clothes.

The Doctor staying up all night caring for a baby that was close to death is just what I think The Doctor would do. His explanation of why he doesn’t do ‘Time Lordy’ things is also very him. When it comes to natural things like illness he really shouldn’t interfere if he can help it. Nature should not be messed with. But, of course, he would never let a baby die when there was something he could do. So in the end he gives in and does the Time Lordy thing.