The deadly flu outbreak in Mexico revives painful memories for Hong Kong and raises fresh fears of a global pandemic. It is vital that the virus be contained quickly. Hong Kong people will be able to empathise strongly with those living in fear in Mexico. Face masks are being worn, schools and public venues closed and public events cancelled; officials are offering advice on personal hygiene. It is all reminiscent of Hong Kong's struggle against the outbreak six years ago of severe acute respiratory syndrome, which left 299 people dead in this city. The spread of this new strain of swine flu, which may have killed scores in Mexico and made more than 1,000 ill, some of them in the United States, could yet turn out to be at least as serious. And it raises similar concerns. Like Sars, this is a mystery virus which spreads quickly - and kills. Hong Kong's Centre for Health Protection, set up after Sars to raise our defences against infectious diseases, is closely monitoring the outbreaks and cases of flu in the city. World Health Organisation head Margaret Chan Fung Fu-chun, the former Hong Kong health chief and veteran of the Sars battle, is overseeing the agency's response to the new threat. Dr Chan was picked for the top job largely because of her experience in combating infectious diseases in Hong Kong. The virus in Mexico will present her with one of her toughest challenges yet. The reminder of Sars is no accident. The lessons of that outbreak have, thankfully, been absorbed far and wide. It took time before city health authorities realised that such extraordinary countermeasures and precautions were necessary. In Mexico's case they have been implemented almost as soon as microbiologists confirmed the role of a new virus in the outbreak. That is a sign of progress. Hope is to be found in the Mexican health minister's claim that the death rate has steadied and the exponential rise in the number of infections has not been as great as feared. But against this, the flu strain is a new mixture of human, avian and swine viruses; most of the dead so far have been aged 25 to 45, rather than the elderly and very young who lose their lives to seasonal flu; and testing is likely to reveal many more cases. For Hong Kong, a hub for people travelling to and from many parts of the world, it is a chilling reminder of the need for vigilance. We must not lower our defences. Our city would seem well prepared, thanks to elaborate measures to protect us from the H5N1 bird flu virus, which continues to kill humans who come into contact with infected poultry in the region, including Indonesia, Vietnam and the mainland. But even those defences have been breached from time to time amid attempts to get around controls on the live-chicken trade, doubts over the effectiveness of bird vaccination and fears that the virus is mutating. The government will be relying on expert advice as to what measures, if any, to take and when. There is no need to overreact, but public safety must be the priority.