It would be funny if it were not so serious. As migra-tory birds carry the avian influenza virus west across Europe, Britain is following in the footsteps of Russia, Ukraine, Romania and Turkey and asking hunters to shoot down as many incoming ducks and geese as possible. They have been issued with bird-flu testing kits to see if their victims are carrying the dreaded virus, but they really have little to worry about: all the cases of direct bird-to-human infection have occurred on family farms in southeast Asia.

The panic over bird flu is not wholly misplaced. If the H5N1 strain that is currently ravaging wild bird flocks learns to pass between human beings easily while retaining even a tenth of its current lethality, the world would face an influenza pandemic as grave as the ?Spanish influenza?, which killed between fifty and a hundred million people at a time when the world?s population was a third of what it is now.

Killer pets

Only in the past couple of decades has it been understood that almost all the quick-killer infectious diseases that have emerged to ravage human populations come from domestic animals. Human beings in the wild, like other predators that live in small, isolated groups, would rarely have fallen victim to the quick-killer viruses whose natural habitat is animals that live in large herds.

Even if such a disease did jump from some prey animal to the hunters who killed it, and even if it then adapted enough to infect the other members of the hunter-gatherer band, the new, human-infectious form would usually die out when it had run through those few people. Only when civilization brought people together in large groups, and those people began living in constant close contact with domesticated herd-dwelling animals, did the diseases begin to adapt permanently to the human species.

Over the past three or four thousand years this process has given us a whole range of highly infectious new human diseases including smallpox, cholera, typhoid, and the Black Plague. Influenza is usually a relatively mild member of this family of diseases, but the flu virus mutates with great ease, and occasionally assumes a highly lethal form.

Time for change

As our population has grown and the volume and speed of travel have increased, we have become more vulnerable to these ?emergent? diseases, but they are unlikely to emerge on a British or even a Russian farm. Eighty years ago the ?Spanish influenza? virus probably made its way from wild ducks into chickens and thence into human beings on a Kansas farm, but modern commercial farming does not involve people and their animals sharing the same living space. Moreover, if some disease does cross the species barrier, its human victims are far more likely to get early treatment.

The places where the style of farming and the density of human and animal populations still favour the easy movement of diseases from animals into people are mostly in Asia, particularly in southeast Asia. As a first step, it would make sense to create a network of trained observers who would report on any unusual disease patterns among the local farm families or their animals.

In the longer run, farmers throughout the region must be encouraged to change their long-established ways of raising poultry, pigs and other animals. That is a tall order, but similar shifts in farming practice have already happened elsewhere, and at least the region?s economy is developing fast enough for it to provide markets for a more commercial style of farming and non-farm jobs for those no longer needed on the land.

The countryside wouldn?t be nearly so picturesque at the end of the process, but the world wouldn?t be facing so many new diseases either.