How should individuals and governments respond to the possibility of new outbreaks?

There is a small but finite chance that a sizeable asteroid is on a collision course with Earth. Most people would agree that the risk is so low that there are more important things to worry about.

What about a possible flu pandemic? It's a difficult call. History suggests we can expect another global pandemic, yet it is devilishly hard to predict when. On the other hand, if one did strike its impact would be immense.

So how does a government respond to a low-risk high-impact scenario? It may seem obvious that drugs and vaccines should be stockpiled and plans put in place to cope with an outbreak. But which drugs and vaccines? Such stocks cost money and somebody has to do all that planning. Is it a good use of time and money when there are so many other priorities? What about other possible challenges - XDR-TB (extensively drug-resistant tuberculosis)? Antibiotic-resistant bacteria? Chlamydia and other sexually transmitted infections?

Governments will seek advice, and tools such as modelling can provide useful input. Yet knowledge will be incomplete and possibly disputed. And other factors will influence decisions - such as a government's political philosophy, economic circumstances and pressure from the media.

Individuals will face different issues. Sales of chicken plummeted when avian flu was discovered in Norfolk, while uptake of the MMR vaccine dropped when fears of a link with autism were raised. It is tempting to call such responses irrational, given the extremely low risks involved.

But they can also be seen as sensible cost-benefit analyses – why take even a small (or perceived) risk if it can be easily avoided and appears to have no consequences? A greater risk will be tolerated if there is a corresponding trade-off in benefits - so the possibility of harm from mobile phones is more than outweighed by their value to us.

In terms of emerging infections, our behaviour will depend on our perceptions (is the threat real?) and the extent to which proposed measures inconvenience us (what will I have to give up?) - and, of course, our individual personalities. Voluntarily avoiding chicken for a few weeks 'just in case' is very different from not being allowed to leave the house.