I once worked on a chicken farm. Actually “farm” is far too gentle a word for the way these chickens were raised, and “factory” sounds too clinical. This was the seventh circle of chicken hell, a clucking, stinking, filthy production line with just one aim: to produce the maximum quantity of edible meat, as fast and as cheaply as possible, regardless of quality, cruelty or hygiene.

The creatures were raised in vast hangars, living on a diet of hormones, antibiotics and cheap grain, thousands crushed together in their own dirt under artificial light, growing from chick to slaughter size in a few grim weeks. (The most accelerated lifespan is now just 40 days.)

That was on a kibbutz farm more than 20 years ago, in the midst of what we can now see as a revolution in livestock production, when science, economics and human appetite combined to forge intensive animal farming on an industrial and global scale.

Those mass-produced chickens were evidently ill. Air had to be pumped into the foetid shed to stop them suffocating. They still died at a pitiful rate, from heart attacks and stress, their bones often too weak to carry the weight of their artificially enlarged bodies. These were “wastage”. The carcasses were kicked into a pile, and eventually removed by digger.

One did not need to be a scientist to know that something very sick was being produced in that shed.

As swine flu spreads, and fear spreads faster, it is worth remembering that this, and other animal-to-human viruses, are partly man-made, the outcome of our hunger for cheap meat, the result of treating animals as if they were mere raw material to be exploited in any way that increases output and profits.

There is a tendency to see a flu outbreak, like the plagues of old, as an unstoppable natural event, a scourge visited on Man from above. But there is nothing natural about this form of disease: indeed, it stems from an abuse of nature.

Vast modern pig farms, like the huge poultry plants across the globe, are ideal incubators of disease, and many scientists believe that viral mutation can be directly linked to intensive modern agricultural techniques. With enfeebled animals packed into confined spaces, pathogens spread easily, creating new and virulent strains that may be passed on to humans. When dense populations of factory-farmed animals exist alongside crowded human habitations, the potential for disaster is vastly greater.

The stress of such vile living conditions makes mass-produced animals more vulnerable to contagion, while the concentration on a few, high-yield breeds has led to genetic erosion and weakened immunity. We have created an environment in which a mild virus can evolve rapidly into a much more pathogenic and contagious form.

Six years ago virologists warned that swine flu was on “an evolutionary fast track”. A US Public Health report last year pointed to “substantial evidence of pathogen movement between and among these industrial-scale operations”. A year earlier the UN food agency predicted that the risk of disease transmission from animals to humans would grow with increasingly intensive animal production.

During the latest bout of avian flu, governments and the livestock industry were quick to blame wild birds and small-scale farms for spreading the disease. With hindsight, it appears that poultry in backyard flocks were markedly more resistant to a virus that has been traced directly to huge factory farms.

Food celebrities such as Jamie Oliver and Hugh Fearnley- Whittingstall have raised public awareness of the way modern meat is produced. But such campaigns tend to focus on the bland taste, ethical or environmental issues such as the toxic waste produced by factory farming, or the amount of water needed to produce a single kilo of beef (16,000 litres).

Far less attention has been paid to the more direct threat to public health posed by industrialised meat production, in which the basics of animal husbandry have been ignored. This, in turn, can be traced to the astonishing transformation in the world's meat-eating habits.

Humanity is more carnivorous today then ever before, thanks to selective breeding techniques, low world grain prices, global distribution networks and the Chinese economic boom. In 1965 the Chinese ate just 4kg of meat per head per annum: today the average Chinese citizen consumes 54kg a year.

The number of animals on the planet has increased by nearly 40 per cent in the past 40 years, but instead of being dispersed across countryside these food units are increasingly concentrated into compact industrial blocks. The number of pigs has trebled to two billion. There are now two chickens for every human.

Industrialised food production has changed the world's diet, providing a cheap and plentiful form of protein. Yet it comes not only at a moral and environmental cost but also in terms of world health: the silent germs mutating and evolving amid the filth.

Factory farming is necessary to feed a hungry world. But doing so without also unleashing new diseases requires far more global co-operation on biosecurity, much tighter international regulation of the meat trade and, above all, a change in the way we produce animals for food. Mass-produced meat can kill you, even if you never eat it.

In 1953 British textbooks insisted that the war against germs had been won by antibiotics, declaring “the virtual elimination of infectious disease as a significant factor in social life”. Accepting that premise, Michael Crichton's The Andromeda Strain imagined the world under assault from a microbe from outer space.

Today the world is once again under attack from infectious diseases. The latest plague does not come from God, or from other planets. It does not simply come from infectious animals and rogue microbes. It also comes from Man.

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