How do flu viruses spread so rapidly across the globe?

Chris - Okay, Les. Very good question and very topical too because we're expecting flu to arrive in the UK at any moment. There are lots and people out there scrutinising what's coming in to testing laboratories all over the country to start logging the arrival of the virus and then tracking where it goes. And the fact is that flu is an infection of humans and that to infect someone, you have to come into contact with a virus that a person has made.

We know where flu comes from. It comes originally from birds and specifically aquatic birds. They infect humans but also other animals like pigs and periodically, humans infect a pig with a strain of human flu, a bird infects a pig with a strain of bird flu, the pig mixes the two together and you produce a new strain, a pandemic strain, that comes back out of the pig, and it goes back into the people.

Actually, how it spreads though is again down to either birds or humans. Now birds can fly and that means that even though people can't necessarily walk across the ground surface more than a certain speed, birds can carry infections very, very quickly because they can fly very quickly. So certainly, some of the pandemics that we've seen in the past probably were accelerated or assisted in their spread by animal movements.

But nowadays, this has been really brought down to be a minor contribution because the dynamics of human populations are absolutely huge.

In the year 2000, the House of Commons working party, a government working group assembled to look at air travel, estimated that at any moment in time, there was about 500,000 people airborne around the Earth at any moment.

和the reality now is that where historically, if you'd wanted to go from say, Sydney, Australia to London, then it would've taken you months to do that journey. And if you were incubating something in Sydney when you left, by the time you got to London, you would either be dead or better. Now because of air travel, you can be on the other side of the world in less than the time it takes you to develop symptoms of an infectious disease which means you can leave, be fully infectious, and infecting people en route, and you won't even know you're ill yet. And so, as a result, these pandemics are being accelerated or the risk of their transmission is being accelerated by modern air travel and we saw this with SARS in a very big way.

We don't think that flu comes from space. There are some people who do think that but we don't think that that's true. We have pretty good idea as to how it spreads, but we certainly have to keep our eye on modern transport because it's a big worry. Because if we aren't very vigilant and we don't watch what people have and where they go with it, and what animals have and where they go with it, then as the population increases, we're going to see more and more risks in this kind of thing happening. You just have to look at what happened with SARS in 2003. That got thousands and thousands of people, 8,000 people in a very short time. At a time when the world was gearing up for weapons of mass destruction release because we thought Saddam Hussein was going to unleash weapons of mass destruction of an infectious nature on the world, and luckily he didn't. But the whole world was prime for that to happen and SARS still ripped through all of that without too much trouble. But it's a really interesting question and thank you for that.