At first sight, it seems strange that the Government should rank influenza as the biggest threat facing the country. After all, flu is a familiar foe. It turns up every winter. And though it does kill some people – very old, very young, or already infirm – the worst that most of us needs to fear is a few unpleasant days in bed.

In an age which has defeated smallpox, and is even steadily beating back cancer, it would not seem to present much of a problem. Wealthier and healthier than ever, we have come to assume that we left the threat of devastating plagues behind way back in history. In comparison with the much-publicised terrorist threat, it seems small beer indeed.

And yet, ministers are right. The danger of a pandemic – which they reckon could kill 750,000 people in Britain alone – is all too real, for every so often, the disease goes though an evolutionary leap. Normally an existing flu virus undergoes a slight mutation, which enables it to infect some people who have built up immunity from previous bouts of the disease. But some three or four times a century, a completely new one arrives, apparently from nowhere. No one has any immunity against it so – if it is vicious enough, and once it has learned to spread from person to person – it is free to commit mass slaughter.

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That is what happened in 1918-19, when a new strain of flu tore through the war-weary world, carrying off 50 million people, including 250,000 Britons. More died from it in their beds than perished on all the battlefields of the murderous Great War itself. And, like all the nastiest pandemic viruses, it targeted the young and healthy, rather than the old and frail, treacherously provoking robust immune systems to overreact so that they kill their owners rather than the virus.

Since then, there have been two much milder pandemics, in 1957 and 1968. Experts agree that another is long overdue, and have been warning that we are unlikely to get off lightly a third time in a row. But until yesterday, they were concentrating on the avian flu that has spread out of Asia to infect birds around the world. H5N1,, is truly scary; 250 people have been killed by it, more than half those known to have been infected. But it has not yet become infectious between people. Scientist worried that it might do so by infecting a pig already harbouring a different, human flu virus, since the animals are susceptible to both: the two could then mix their DNA, creating a deadly new strain that could pass from person to person.

But even at the height of concern about H5N1 four years ago, some experts were warning that the new pandemic might emerge from elsewhere. And this could be what has just happened. The latest virus, A/H1N1, that is rapidly spreading through North America is new and contains a mixture of bird, pig and human strains. Early reports suggest that it takes most of its victims among healthy 25 to 45 year olds.

The World Health Organisation says it has "pandemic potential", but nobody yet knows whether it will indeed sweep through the world and, if it does, how deadly it will be. The bad news is that it is related to the H1N1 strain that caused the 1918-19 plague. But there is good news in that too, for the remnants of that virus hung around into the 1950s, which may give the over 50s some protection.

If it does decide to spread, it will do so quickly. In 1918-19, flu chugged across the world at the speed of steam, taking five or six months to become universal. Now, like a jet plane, flu can circle the world in little more than a day. And once out, as it is now, nothing can stop a determined virus. Checking airports for infected passengers, as Japan is doing, is a waste of time: victims are infectious for several days before they show any symptoms.

But again, there is ground for hope. Flu, experts say, normally tends not to spread in the summer; it prefers autumn and winter weather, passing between children as they return to school. Partly because of a vigorous campaign by this newspaper over bird flu, ministers tardily decided to follow other developed countries by amassing big stocks of Tamiflu and Relenza, two drugs that seem to work against the new strain.

And even if it does spread, we might, conceivably, get lucky for a third time, should it turn out to be a mild strain; because two pandemic viruses never seem to rage at the same time, it might keep H5N1 at bay.