Did you intend the film to be quite so scary?
Yes. It's this idea of a psychological virus. Like road rage, that moment when the red mist descends, if you magnify that a thousand times and experience it to the exclusion of everything else, imagine what that kind of power would be like. That was our monster. If you really think about that, it's very, very scary indeed.
We cast athletes as the monsters, because they're incredibly powerful and you can sense that power in them. We also used manifestations of Ebola: the vomiting and blood pouring from orifices; and the rictus look of fear at the hydrophobia stage of rabies. Hopefully all of that makes it a very scary film.
What was the biggest challenge you faced?
The London stuff at the beginning of the film was the biggest challenge. Although we didn't have much money, we wanted to say: here's a big film with a large agenda and with a huge landscape to look at.
One of the problems with filming in Britain is that there's this slightly depressing air of familiarity and scale, but we wanted to make it something mythic and give it a mythical size. We didn't have enough money to pay for carnage on a city-wide scale, but the idea of an emptied London gave us the iconic image we needed.
How has the recent Anthrax hysteria affected the way you now view the film?
For the people who actually suffered Anthrax, it was obviously terrifying. For everyone else, it was that psychological thing of wondering if you were going to get it every time you opened a package. Again, it's that idea of putting the virus in people's minds which makes our film feel so contemporary. Just like zombie movies reflected on a paranoia about nuclear war, I think this film will give us all something more to think about.
Would you be up for directing Irvine Welsh's "Trainspotting" follow-up, "Porno"?
I've just finished reading it, funnily enough, and I really enjoyed it. I found it very difficult to start with, because I am in such awe of the original book, which I think is a modern "Ulysses". I think the idea of taking the characters ten years on is brilliant, but one of the problems at the moment is that the actors - Ewan and Ewen, Bobby, and Jonny - don't look ten years older. It's a fantastic idea, I don't know whether it will happen or not because of the logistical problems that would be involved. I'd love to go back to it, I think the idea of a ten-year gap is a great idea.