Experts yesterday expressed concern that the return of the human form of H5N1 flu means the strain may have found a faster way of mutating to cross the species barrier. If this is so, H5N1 could be a potential pandemic strain, some experts said. While initial genetic analysis showed that the latest strain was a pure avian flu virus, some said answers needed to be found quickly on how and why the boy who went to Fujian province had become infected. 'The question is how was it transmitted? Were the family members all exposed to one source?' one international expert asked. Another said the boy probably became infected while in Fujian. University of Hong Kong virologists said the internal genes of the H5N1 virus found in the boy did not have a 'human flu component', pointing to a pure avian flu. Legislator Lo Wing-lok, who represents the medical sector, said this meant the virus' behaviour would be similar to the 1997 H5N1 that jumped from a bird directly to a man, and therefore could not efficiently transmit from man to man. 'If it is human-to-human transmission it will be an even bigger cause for concern,' he said. 'If it is a bird-to-human virus we will be closer to that of the 1997 virus. By controlling the poultry we can stop the outbreak in humans, which we did in 1997. 'If there is human-to-human transmission it may be more difficult to control. It will be like any [form of] flu.' Dr Lo said an investigation should also centre on the H5N1 flu case coming in the midst of an outbreak of pneumonia-like cases in Guangdong. Mainland doctors should be told how to take swabs properly from patients suffering from flu in the early stages of the disease. 'The situation in Hong Kong and Guangdong is a mixed bag of different viruses and bacteria happening at the same time,' he said. 'Unfortunately, we are in the flu and pneumonia peak season. Who is to say that some of the cases are not due to bird flu?' Experts have warned that the world is ripe for the next flu pandemic, which would occur if a new flu strain is able to infect huge numbers of people who do not have previous immunity to the flu. Guan Yi, associate professor of the University of Hong Kong's department of microbiology and the gene sequencer of the bird flu, said it was still too early to say how the new H5N1 strain would behave or even assess its ability to spread in humans. 'But I do not think we have any evidence to show that this virus came from the mainland,' he said. The 2003 strain was different in its internal genes from the 1997 virus. Fred Li Wah-ming, who chairs the Legislative Council food and environmental hygiene panel, said he did not think the latest H5N1 discovery had anything to do with this month's outbreak of pneumonia-like infections in Guangdong. But he said he was worried the killer flu virus of 1997 had returned. 'We have to step up measures. All hospitals and clinics have to be alert to different symptoms of the flu,' he said.