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As scientists race to unravel the mysteries of SARS, one issue high on their agenda will be the likelihood that the new virus is a cross-species transmission in which the virus has mutated from its animal carrier so that it can infect humans, who have no immunity from the alien invader. The most obvious examples of this are HIV and influenza, and the latter disease has disturbing parallels with SARS. The flu virus lives usually in the stomachs of waterfowl, and the two are co-adapted — the birds don't get sick. It is widely believed among virologists, however, that with the domestication of ducks in southern China 2,000-3,000 years ago, flu jumped species.

This region has always had high densities of people living in close proximity to large populations of pigs and chickens. It's not known in which order, but with this ready pool of targets near at hand, flu has transferred from ducks to all three species — and once established, it can swap back and forth between its different new hosts with devastating effect. The virus survives and thrives by constantly mutating — so that just as our immune systems recognize and kill off one strain, a new one emerges against which our defenses don't work. Most are minor adaptations, the product of genetic "drift." Every now and then, however, something more dramatic occurs: a genetic "shift." Also termed "a reassortment event," this is the creation of a wholly new strain with genetic elements taken from viruses found in different species.

In 1997, Hong Kong's "bird flu" was a virus that was part human, part avian. Much luck, hard scientific labor and prompt containment measures prevented that outbreak from turning into a global catastrophe. Next time we might not be so fortunate. Medical records dating back to the 18th century show waves of influenza rolling westward from Asia through Russia into Europe with disturbing regularity. Three or four times a century, a pandemic spreads from flu's heartland. So statistically speaking, since the last reassorted strain emerged in Hong Kong in 1968, we're due for another one.

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Every pandemic is calamitous. The "Russian flu" of 1889-90 is thought to have killed 250,000 people in Europe alone. No epidemic, however — not even the Black Death of the Middle Ages — compares in mortality to the "Spanish flu" of 1918. Around the world, 40 million died of it within one year. Unusually, the 1918 flu did not come from Asia. The first outbreak began at a barracks in Kansas in the spring. The second, most virulent strain of the disease emerged simultaneously in September in Boston, Massachusetts; in Brest on the Atlantic coast of France; and in Freetown, the capital of Sierra Leone.

The unusual origin, spread and potency of the 1918 flu are almost certainly connected to World War I, which saw large numbers of men crowded together in camps, transports and trenches. We don't know exactly why this particular strain was more deadly than others — though scientists may be closing in on the answer — but we do know that it caused a global disaster. In Philadelphia, 7,500 people died of it in two weeks. The supply of coffins ran out; streetcars were used as hearses. Families lay dying in their homes, unable even to stir to feed themselves. Some lingered in delirium for weeks, coughing foamy, blood-tinged sputum, while others were dead within 24 hours.

We cannot be sure whether the next pandemic will be this bad, any more than we can be sure when it will come or where it will start. There is only one thing of which we can be certain: that it definitely will happen again. We can also be sure that, as so often before, it very likely may begin in southern China. Variants of the strain that caused the "bird-flu" outbreak six years ago are still cropping up around the region, or the outbreak could arise from some other strain altogether.

Professor Kennedy Shortridge of the University of Hong Kong has run blood tests on birds, animals and people in the territory, in Taiwan, in Jiangsu province and in the Pearl River Delta. Especially in the latter two places, he found farmers with antibodies suggesting exposure to every single type of flu that exists in other species. So it's just a matter of time before one of those types adapts to human beings and takes off.

In the meantime, we wait to see if SARS can adapt with the same deadly efficiency as influenza — and once a virus achieves airborne transmission from one person to another, the consequences might be as brutal as the 1918 flu that killed one in 60 of all the people on earth. Perhaps if we knew that SARS had come from another species, we could identify how it had changed and we could design drugs or vaccines to tackle it. By the time we had produced them, however, the disease would already have done its deadly damage. Once again, we find ourselves at the mercy of nature.

• Pete Davies
• South China has long been a breeding ground for infectious disease — and this time, the world could pay the price