Bird flu has, unfortunately, returned to Hong Kong, just four months after live chicken sales resumed. Yesterday's culling of 19,000 poultry at the wholesale market in Cheung Sha Wan is hardly appetising news in the new year. But like it or not, the disease associated with live poultry sales has become a regular occurrence. Unless we are prepared to change our eating habits, the culling - and compensation to traders for their loss - will happen again. The confirmation of the H7 virus found in samples of mainland-imported chicken came after Hong Kong reported its first avian flu case this winter. The 68-year-old woman, who travelled to Shenzhen earlier, was still in critical condition at Tuen Mun Hospital yesterday. Separately, the Guangdong health authorities have also reported the first case in Shenzhen this winter. So far there is no evidence to show the two incidents are connected. But they come as a chilling reminder of the threat to public health during the flu season. The government has rightly activated the necessary steps to contain the spread. Announcing the test results in the early hours of yesterday, Secretary for Food and Health Dr Ko Wing-man said the authorities would strengthen surveillance along the supply chain, be it local or from the mainland, under the current mechanism. Our defence system has been tried and tested. The measures put in place are based on the lessons learned from repeated outbreaks since 1998. But as far as the fight against infectious diseases is concerned, there is only room for vigilance, not complacency. That we ushered in the new year with another mass poultry slaughter shows we cannot be complacent when it comes to public health. We need to guard as much against the bird flu virus as a false sense of security arising from our enhanced mechanism. At stake are more than the economic costs and the disruption to fresh poultry supply. The public should seriously consider supporting an end to live poultry sales lest bird flu haunts the city again.