Only two things keep Sir Liam Donaldson, the Government's chief medical officer, awake at night, he confided in a recent interview. A terrorist attack and avian flu.

We have had the first and the emergency services responded in exemplary fashion, in large part due to the meticulous planning and preparation led by Sir Liam. The head of one major A&E department at a London hospital boasted after the 7 July bombings that they could have coped with twice the number of casualties.

So far so good. But while the threat of terrorism has now been brought home in the most vivid and shocking way, the threat posed by nature's terrorism - its capacity to manufacture lethal viruses that can spread round the globe decimating populations - is still under-recognised. Think of Aids and the way, for more than a decade, the West turned a blind eye to the devastation unfolding in sub-Saharan Africa.

Now a new threat is lurking on the horizon - avian flu - and it is surprising how little it has penetrated the public consciousness. Last month I discussed the issue with a group of leading medics who took the view that the Government was sensibly taking precautions but the threat was quite low.

This is certainly not the case. The reason why the threat is very real now can be understood with the aid of simple mathematics.

Ordinary winter flu of the kind that lays thousands of people low every year in Britain is one of the most infectious diseases known. Ordinary flu causes only mild illness in healthy people, though it can be fatal to the elderly and vulnerable.

The worry now is that the flu virus, which undergoes subtle mutations each year leading to a new outbreak, could be subject to a major mutation brought about by its mixing with the flu virus that infects birds, avian flu, which could cause a new pandemic.

There were three flu pandemics in the last century - in 1918 (the worst), 1957 and 1968. The mutations that led to these pandemics occurred randomly so the chances of another major one happening depend on how much human flu virus is mixing with bird flu virus.

This is a bigger risk in the Far East because many families there raise chickens and ducks, often living in close proximity to them. When the last flu pandemic occurred in 1968, China had a human population of 790 million and a poultry population of 1.23 billion. Today there are almost twice as many humans and ten times more chickens.

Similar changes have occurred in other Asian countries, creating an incredible mixing vessel for viruses. It is as if the number of people playing the National Lottery had risen tenfold - raising the chances of a winner hitting the jackpot by a similar amount. As for jackpots, so for lethal mutations - ones that could trigger a fatal illness that would sweep around the globe.

Our only defence against avian flu is the anti-viral drug Tamiflu, 13 million doses of which the Government is buying over the next two years. It won't cure the illness but it should lessen its severity if it strikes. A vaccine is under development but it would only offer partial protection.

Next month the Government is to mount an exercise to deal with a potential pandemic, co-ordinated by the Cabinet committee that led the response to the bombings on 7 July. From wherever the threat may come - suicidal humans or deadly viruses - we do well to be prepared.