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One puzzling question about the swine flu outbreak in Mexico and the US is why the same virus appears to have had such different effects in the two countries.

In Mexico, young adults aged 20 to 40 are the main victims and the virus has apparently struck with lethal impact, causing up to 149 deaths.

Yet in the US it appears to be causing mild illness, mainly affecting children, and there have been no deaths. Scientists at the Centre for Disease Control in Atlanta, US, yesterday reported having successfully sequenced the Mexican virus and confirmed it was the same as the US version. So why the difference in impact?

One possibility is that the virus has spread much further in Mexico than has so far emerged, but has only just established a foothold in the US. Tens of thousands of people may have had flu symptoms in Mexico but not reported them while only a few score of people may have been infected in the US, which, with its superior surveillance systems, may have detected the unusual nature of the virus sooner.

If that is the case then the number of deaths in Mexico might not be out of line with that seen in other flu outbreaks. Equally, the absence of deaths in the US (and elsewhere) might reflect the low numbers infected there.

It could be "a numbers game", as Dr John McCauley, a flu expert at the National Institute for Medical Research in Mill Hill, north London, puts it. We know the number of deaths in Mexico but we don't know the number infected. Until we do, we can't assess the severity of the threat to the rest of the world.

It is one of many unknowns about this virus, whose precise combination of pig, bird and human genes has never been seen. What we do know is that it is novel, spreads readily from person to person and it kills. That is enough to put the world on alert.

But there is much still to learn before we batten down the hatches and dose ourselves with Tamiflu. This novel strain of swine flu virus has the potential to cause a pandemic – on that much scientists are agreed. However, it does not necessarily mean the end of life as we know it.

Of the three flu pandemics in the last century, the first in 1918 was by far the worst, infecting one-fifth of the world's population and causing 20 to 40 million deaths. It was also caused by an H1N1 type virus like the present outbreak, and was thought to have originated in pigs though subsequent research has suggested it may have come from birds.

The two subsequent pandemics in 1957 and 1968 were very different, causing one to two million deaths around the world and 50,000 in the UK. Although the disease they caused was severe, cities did not empty, travel did not come to a halt and economies were not devastated. Life continued much as before.

This time we are better prepared with stocks of antiviral drugs and an emergency plan developed over the past five years. One of the biggest challenges, if the flu does hit in a major way, will be delivering the antiviral drugs to those that need them within the 48-hour time window necessary to ensure maximum effectiveness.

In the UK, ordinary winter flu kills between 4,000 and 20,000 mainly elderly people each year. In the last serious epidemic year in 1996/97 the toll rose to 35,000 deaths.

Even if the swine flu outbreak develops into a pandemic, most of us will live to tell the tale.