Global health experts meeting in Toronto to discuss Sars are puzzled about the role of the coronavirus widely believed to cause the disease. The head of Health Canada's microbiology lab, Frank Plummer, said it was puzzling that only 40 per cent of Sars patients had tested positive for the coronavirus. But it had been found in specimens from 14 per cent of people with suspected Sars who were not sick and demonstrated no symptoms of the disease, with some - but not all - having travelled to Sars-affected countries. 'They're an interesting group and of concern because those are individuals who don't meet the clinical case definition of Sars,' Dr Plummer said. 'That may mean there's a fair bit of mild illness caused by this coronavirus that we're not recognising. And I think it will mean it will be much harder to control with quarantine and isolation if the coronavirus is the whole story.' Paul Gully, also of Health Canada, said Sars symptoms varied widely in terms of the types of symptoms and the time of onset. In Hong Kong, medical sector legislator Lo Wing-lok said some people who have contracted coronavirus might develop antibodies and become immune to the virus afterwards. As for those patients who tested negative on coronavirus but still develop Sars, Dr Lo said it might be due to various reasons, such as the accuracy of the test and whether another virus had contributed to developing Sars.