If the Sars battle taught us anything, it was that timely reporting of information is a key to limiting the damage that can be caused by outbreaks of infectious disease. We are still learning new details about what information health officials in the region had, when they had it, and what they chose to do with it. The picture that has emerged in southern China, however, is one of early detection and intervention - but delayed sharing of data. Cases were known to Guangdong health authorities by early January, and the national Centre for Disease Control (CDC) was notified later that month, but this did not reach Hong Kong health agencies until much later. It was Sars that prompted a series of pledges from the public health chiefs of Hong Kong, Guangdong and Macau on more direct communications and an expanded list of diseases that would trigger notification. Now, just two months after the first of those pledges, and even as the fight against Sars continues, we have a test of those co-operative intentions, in the form of the encephalitis outbreak in Guangdong. As we report today, 211 children have now been infected and 18 of them died. Yet Hong Kong's Department of Health did not know about it until the outbreak hit the press, an indication that health agencies are not cutting through the red tape quite fast enough and that the definition of what is notifiable may not be broad enough. In principle, all sides have agreed to expand the list to include HIV/Aids, dengue fever, influenza, tuberculosis and malaria. But there is little indication that reporting will include emerging infectious diseases that are not yet known and therefore cannot be on any notification list. This would seem to be crucial for our region, where heavy traffic in people and goods - and global connections - can help transform a local health threat into a global epidemic in a very short time. The challenges standing in the way of streamlined cross-border reporting of disease information cannot, of course, be underestimated. On the mainland, a recently established CDC is struggling to set up clear lines of communication with local hospitals and provincial health authorities. Military hospitals and national health authorities have for years treated health information as sensitive data, and changes in these attitudes will not happen overnight. In Hong Kong, much of the medical community's energies are still being diverted to control and treat Sars. Moreover, a post-mortem examination on how the health system responded to Sars may bring changes in how it is structured - and see the introduction of agencies to monitor infectious diseases. The importance of liaison must remain central, even as the two systems evolve. Public health in the region, and the world, depends on it.