Each Sars patient was responsible for infecting 27 people in 30 days. Every bird flu infection will cause 1,024 others in a month. That's 38 times as fast a spread of disease. It is one of the frightening assumptions underlying a disease model that experts hope will help prepare for a H5N1 pandemic. With a worst-case scenario as gloomy as that - it is based on the Spanish flu of 1918, which killed 50 million people - the experts say, not surprisingly, that the first two weeks will be key to containing a bird flu outbreak. The Centre for Health Protection's disease modelling committee is assuming the first wave of a bird flu pandemic will last three to four months. Under its worst-case scenario, the disease-modelling team from the University of Hong Kong assumes each flu patient will infect two others every three days. 'According to this formula, each index patient will infect 1,024 people in 30 days,' said the committee's chairman Gabriel Leung, HKU associate professor of community medicine. In 2003, Sars infected 1,755 people in Hong Kong, of whom 299 died. Dr Leung said based on the current bird flu outbreak in Southeast Asia, in which more than half the 120-plus people infected have died, the first two to three weeks would be crucial to putting a lid on the spread of the virus. Dr Leung said his team would work with the Centre for Health Protection around the clock if there is a pandemic. 'We will update the situation in our modelling to get a real-time projection of the number of infected, hospital admissions and mortality rate. The projection will give us an idea when and how to act - for example, the right time to quarantine the infected or distribute [antiviral drug] Tamiflu to the health-care workers.' Dr Leung said the government had to weigh the costs and benefits of the measures it took. 'For example, quarantine and contact tracing are standard procedures, but when we have tens of thousands of cases, are there enough holiday camps to quarantine so many people? And do we have manpower for so much contact tracing?' The centre estimates a flu pandemic would attack a million people in Hong Kong.