For years, we've been haunted by the spectre of bird flu, which was first known to kill humans here in Hong Kong, in 1997. That spring, 4,500 chickens died on three Hong Kong farms, and scientists who investigated found a new strain of H5N1 avian influenza - which has since become known as 'bird flu'. Then, in May, a dying three-year old boy was found to be infected with the same flu, which would kill five more people in Hong Kong.

The city called in international experts. They included Robert Webster, whose research had revealed birds may carry flu viruses linked to those that threaten people. He led researchers who found bird flu in poultry markets and over a million chickens were slaughtered, supposedly halting the onset of a global flu pandemic.

But of course, this was not the end of bird flu. In May 2001, there was a further outbreak in Hong Kong, and the government ordered the slaughter of all birds in local poultry farms. The next year, bird flu returned, affecting poultry but also killing ducks, geese and egrets in Kowloon Park and Penfold Park. Swiftly, migratory birds were blamed for bringing the disease - even though most victims in both parks were captive waterfowl, and the few wild birds were egrets that likely lived there year-round.

Bird flu hit the big time in 2003 and 2004, spreading widely in East Asia, from Japan south to Indonesia. There were more human infections and deaths. More dead wild birds were found to have been infected with bird flu - and migratory birds were widely fingered as the major carriers. As a birdwatcher who had become fascinated by the nightmarish potential of a flu pandemic, I began seeking information whilst trying to show the dates and locations of the bird flu outbreaks were very different to bird migration patterns.

Poultry disease specialist Dr Carol Cardona felt it was more likely the flu was spread by humans and e-mailed me saying: 'This particular variant is unusual both for the fact that it can infect humans but also because it can make wild birds, especially ducks, sick ... in my experience sick and dead ducks don't fly far.' This seemed to be common sense, yet common sense was all too lacking with bird flu.

As bird flu's spread continued, so too did blaming migratory birds as carriers. Hysteria mounted, fanned by outlandish predictions and absurd pronouncements. UN official David Nabarro claimed bird flu could kill up to 150 million people. Wild birds were no longer feathered friends, but instead could bring, 'Death raining down from the sky' (Time), or become 'The epidemiologic equivalent of intercontinental ballistic missiles' (Wall Street Journal).

Here in Hong Kong, legislator Tommy Cheung Yu-yan once suggested giving each person a gun, so that on seeing a migrating bird we could shoot it down.

Amid the hoopla, US science writer Wendy Orent e-mailed me. 'I agree totally with you that migratory birds can't be the chief culprits,' she wrote. 'As evolutionary biologist Paul Ewald has argued for years, among wild birds avian flu strains will have to evolve towards benignity; virulent flu will be eliminated eventually in the wild.'

Until then, I'd only learned of flu viruses changing through mutations, or mixing DNA between strains. Yet to Ewald, diseases including flu evolve according to Darwin's principles of natural selection.

Previously, Ewald had looked at variants of human flu, which are normally relatively benign, but from 1918 to 1919 included a form that caused the worst pandemic in history. From his evolutionary perspective, Ewald believed a crucial fact is that flu must be transmitted from a living person. Normally, this means someone who can walk around, coughing and sneezing, so typical flu kills only a small percentage of those infected. But for the severe pandemic, there was a special situation: trench warfare.

Ewald reasoned that during the first world war, millions of men were fighting in conditions that meant even very sick individuals could transmit flu to others - and flu responded by evolving to a highly virulent form. Known as the Spanish Flu, it killed at least 40 million.

In Ewald's view, wild birds must also be able to move in order to spread flu - which explains why ordinary bird flu strains are mild. But in densely packed poultry farms, very sick birds can infect others, so highly lethal forms of bird flu evolve.

Evolutionary biology can explain several other aspects of flu that might otherwise seem mysterious. For instance, the Asian Flu and Hong Kong Flu were two pandemics that originated in this region in 1957 and 1968 - coinciding with periods when Mao Zedong ordered people to live in regimented communes and was overseeing the craziness of the Cultural Revolution, which even included 'therapy' with injections of chicken blood.

Yet it seems that for too many people involved with bird flu, this is an inconvenient theory. Companies running industrial-scale poultry farms have little interest in admitting their role in evolving flu strains deadly to birds. Various flu researchers seem wedded to notions the next big pandemic could be moments away. The global toll from bird flu may be around 350, yet we should still be very afraid: major funds are still needed, apparently, for projects that include 'conservationists' assessing birds as deadly flu carriers, though evidence is sketchy at best.

But to answer the earlier question: half of us are not dead from bird flu as the social conditions required for such a flu do not exist today, and surely can never exist. And there is little to fear from bird flu - unless, of course, you're a chicken.

Martin Williams is a writer and photographer, specialising in wildlife and conservation, and has led pioneering studies of bird migration on the east China coast. He holds a doctorate in physical chemistry.