Why did they have to kill all the chickens. The chicken farmers say chickens have always had bird flu during winter and they were not culled in the past. Something changed in 1997. Until then, the virus had no known effect on humans. But in May that year, a little boy played with chicks in his classroom. His teachers brought the baby birds to school as a way to help children learn about and care for living things. When he became ill a few weeks later with a high fever and flu symptoms, no one thought it had anything to do with chickens. Although baffled by his disease, none of the hospital doctors thought about chickens either. Because at that time chickens didn't give humans this kind of disease. Sadly, the boy's health deteriorated and he died before anyone could work out what kind of infection had invaded his body. But he left a very important legacy. Someone at the hospital had the good sense to take a throat swab and to send it off for virus testing. And when the laboratory detected a virus no one could identify, it was sent off to experts both at home and abroad for further examination. Eventually it was identified as an H5N1 virus, a type only seen before in birds. Alarm bells began ringing in the minds of those who know all about the influenza virus and its ability to cause devastating epidemics with high death rates if it changes into a form the human body has not met before. The H5N1 virus is a bird infection, not a human infection, but by chance it was able to survive and multiply in the boy's body. During its incubation, the virus 'borrowed' human flu genes to help it on its way. Had the virus started to pass from human to human it could have developed into a major epidemic. Because the virus is one that human immune systems have no ability to fight, the crucial step of acquiring human genes would turn it into a killer on a par with ebola, anthrax or any of the infectious microbes we so fear. In November 1997, the medical profession's worse fears became a reality. Over the next couple of months, 17 people developed symptoms and had positive tests for H5N1 infection. Five of those, who were among the first to contract the illness or who took a while to get to hospital, died. Fortunately, those who received immediate medical treatment survived, thanks to one drug, amantadine, which was able to kill the virus. The virus taken from each person was carefully examined and checked for evidence it was adapting itself into an effective human virus. So far it seemed it had not gone very far and was still essentially a bird virus. But because more of the bird virus was jumping into humans, a decision was taken to remove the source. Every chicken in Hong Kong was killed during the last week of December 1997. It was a drastic decision that led to horrendous scenes of slaughter and mayhem, but it did stop the bird flu in its tracks. And that is why, whenever the chickens show signs of bird flu, all of them are killed. It is why the markets have to be cleansed rigorously and why the health authorities are talking of closing them more often for cleansing. Yes, it hurts the industry. But if the bird flu ever gets really going again, there will be no one buying chickens anyway. What can we all do to prevent or fight bird flu? First take the threat seriously. Secondly, do all the usual things to strengthen our immune systems during cold weather - get plenty of sleep, eat fresh, unprocessed foods, especially those high in vitamin C, and avoid overcrowded and poorly ventilated places.