The ultimate weapon of mass destruction

The threat to this country from a pandemic caused by bird flu or some such virus is greater than the threat from international terrorism in the view of Gordon Brown and his Downing Street advisors. This is why pandemics are ranked alongside terrorism and other major global ills in the National Security Strategy unveiled last month.

On the face of it, the briefing seems alarmist. The World Health Organisation has recorded only 238 deaths proven to be from a human variant of the H5N1 virus. Yet scientists and security specialists are more in agreement on the risk of a pandemic than on climate change and global warming.

The argument is based on a hypothesis, a short-odds scientific bet. Sir David King, who's just stepped down as the UK's Chief Scientific Adviser, puts it like this: "It is more a question of when rather than if the H5N1

virus mutates into a virulent human form. So far we have noted more than 20 mutations of the virus in only a few years."

His view is supported by every public health official or doctor I have met in recent months. Only last week, three former heads of the Defence Ministry and the Joint Intelligence Committee told me without hesitation that they thought the PM was right to flag the potential menace.

The H5N1 virus may have dropped out of the news headlines just now, but it hasn't gone away – far from it. Last week Egypt recorded the 21st human death from avian flu in about a year. As with a quarter of all human cases, the virus had been passed between members of the same family.

The Lancet has just reported that a 52-year-old man in Nanjing, China picked up the virus from his 24-year-old son, who caught it after visiting a poultry market. The old man survived thanks to the prompt application of antiviral medicines.

Currently there are major outbreaks of the virus in the bird populations of China, India, Bangladesh and South Korea. In Pakistan