Bird flu is causing concern among international health authorities because the world is ripe for a flu pandemic, according to the World Health Organisation's flu expert. Dr Klaus Stohr, project leader of the organisation's Global Influenza Programme, said yesterday the three pandemics in the last century occurred in 1918, 1957 and 1968. 'The last pandemic was more than 30 years ago and the chances of a pandemic occurring are increasing by the day,' he said. When a three-year-old Hong Kong boy died of the H5N1 bird flu in May 1997, it sent alarm bells ringing around the world. It was the first time the flu type had jumped the species barrier and did so directly from birds instead of via a host such as a pig. The virus lay low for a while but resurfaced in November that year, eventually infecting another 18 people, of whom five died. It was halted after Hong Kong took the unprecedented step of culling at least 1.4 million chickens and poultry. 'Since 1998 no humans have been infected by H5N1. The organisation is particularly concerned if humans are infected. We remain very vigilant,' Dr Stohr said. Control and surveillance systems were important in preventing a pandemic strain from spreading, he said. 'We all hope that the 1997 bird flu is the last. Biology is unpredictable but, with the information that we have, we understand that the measures taken in Hong Kong are aimed at controlling the disease and checking and preventing its spread to humans.' Professor Malik Peiris, of the University of Hong Kong's microbiology department, said the surveillance system was able to pick up 'precursors' of the H5N1 virus before the strain mutated to the deadly human form. 'The primary objective from the public health point of view is to prevent a repeat of 1997,' he said. Professor Peiris said the market rest day, implemented on the 25th of every month since July 25 last year, was a crucial step in reducing the presence of the virus in wet markets. In 1997, studies by the university's Professor Kennedy Shortridge found 20 per cent of chickens had been infected by flu. Professor John Tam Siu-lun, of Chinese University's department of microbiology, said Hong Kong had probably 'one of the very best' surveillance systems for bird flu in the world. 'But avian flu will occur every now and then despite all the controls. We still have live chickens in the market,' he said.