About this time four years ago, the Sars epidemic was gaining a foothold on the mainland but was unknown to the outside world. This was because mainland officials initially covered up the outbreak of a highly contagious new killer virus while doctors tried to contain it. The tragic consequences that were to follow as travellers spread the disease to Hong Kong and as far away as Canada remain fresh in people's memories. Because of this disaster more than any other reason, transparency and a fast response have become the bywords of plans to prevent or mount effective counter-measures against similar outbreaks. For example, in the event of the merest suspicion that the H5N1 bird flu virus has jumped the species barrier and become infectious between humans, immediate public disclosure and reporting to the World Health Organisation is essential if an outbreak is to be contained before it becomes a pandemic. The same rules of openness and action apply in the case of suspicious, unexplained illness or deaths, whether among humans or animals. It is for this reason that Hong Kong's Centre for Health Protection has acted quickly to launch an investigation after it was revealed that two Hong Kong pig farmers had died from acute pneumonia caused by an unknown infection. The two men, who farmed at Man Kam To and Yuen Long, died six months apart, on June 4 and December 13. Their deaths were not reported to the centre, which was alerted by media reports. The investigation also follows news of the death from pneumonia on Tuesday of a Guangdong pig farmer. Again, doctors have yet to establish the cause. The centre will carry out tests on samples from the two Hong Kong men in the search for clues to their fatal illness. It has ordered hospitals to report similar cases and set up a hotline for people to come forward if they know of earlier cases. Given that we are talking about three widely separated, apparently unconnected, deaths from a common infection in a very large population, some may find the high-profile response likely to give rise to unnecessary public alarm. That would be unfair to the appointed guardians of world-class public health standards. As health experts have warned, it is essential to track down the cause of these similar deaths among pig farmers quickly and avoid another outbreak like Sars, which also began as a series of unexplained cases of pneumonia. Although doctors cannot rule out the possibility that the cases are connected, they say the chance of contracting pneumonia from pigs is small. However, bearing in mind that Sars also originated from animals, experts have urged the government to do everything possible to establish the cause quickly. Happily, the lessons learned from the challenge thrown down to our health system by Sars have not been forgotten. The threat posed by H5N1 bird flu has tended to recede in the public mind. This is not necessarily a bad thing so long as the worldwide panic-buying of Tamiflu tablets - a treatment for severe flu symptoms but not a cure - has not been followed by complacency. Unfortunately, this is a risk that arises from 'bird-flu fatigue', caused by repeated warnings of a human pandemic that has not happened. The warnings are still valid and not to be taken any less seriously. We are reminded of this by recent news items, including fresh outbreaks of bird flu among poultry in South Korea and Vietnam, more deaths among people infected by poultry in Egypt, and an Indonesian investigation of 22 deaths in Jakarta over a two-month period from an unidentified illness characterised by high fever. Since 2003, Beijing has adopted measures to handle communicable diseases more transparently. But it still tends to regard data on outbreaks as state secrets until they are officially released, and thus not available to foreign scientists. It is hoped that new WHO chief Margaret Chan Fung Fu-chun, a former Hong Kong director of health who had Beijing's backing for the world job, will help smooth out China's remaining credibility problem.