The United Nations' point man on bird flu said a human flu pandemic is nearly impossible for the world to imagine but warned that when HIV/AIDS first emerged two decades ago, few believed it would eventually kill millions. "I remember how many people said, 'That's impossible' and how difficult it was ... to conceptualize at the time," Dr. David Nabarro told The Associated Press in Hanoi on Monday. As bird flu spreads among poultry stocks, most recently in Turkey and Romania, Nabarro urged that preparations be made for a worst-case scenario. Most human bird flu cases have been traced to contact with poultry, but experts fear a genetic mutation could allow the virus to spread easily between people, possibly sparking a global pandemic. He said it's impossible to know what would happen to societies if millions of humans suddenly became sick and started dying. There hasn't been a killer bug on that scale since 1918 when the Spanish flu killed up to 50 million people and sickened 20 percent to 40 percent of the world's population. But Nabarro said it's important for countries, communities and the public to pay attention to researchers' warnings now so the world can try to prevent a flu pandemic that could skip across borders and oceans, possibly causing "billions, even trillions" of dollars in damage. "We don't have a simple way to reassure people, but at the same time, we also shouldn't make people feel any kind of false comfort," he said. "We have to find the right blend between some fear and some sense of real preparedness and willingness to respond." Nabarro said the world needs at least another six months to a year to prepare for a possible flu pandemic. He plans to ask the international community to help finance emergency planning in poor Asian countries that have been hit by the H5N1 bird flu virus. More than 60 people, mostly in Vietnam, and millions of birds have died in the region from the disease since late 2003. He said an agreement about the stockpiling and control of antiviral drugs like Tamiflu, produced by Swiss-based Roche Holding AG, also needs to be reached among countries to guarantee the poor will have access to medicine. "It will be most unfortunate if the stocks that are available are sequestered and they're inaccessible to the people who might need them because the pandemic has started and they can't get them because their community is poor," he said. Earlier this month, UN Secretary-General Kofi Annan suggested patent rules might be suspended in an outbreak to allow other companies to make generic forms of Tamiflu. In recent days, a company in India announced plans to do that. Nabarro said he supported Annan's statements but stopped short of saying Roche should be forced to release control of its patent. Appointed to the U.N. position last month, Nabarro is on a fact-finding mission to Thailand, Vietnam and China. He said the world got a small taste of how dramatically an outbreak could affect everyday life during the 2003 SARS outbreak when the virus jumped rapidly from Asia to other parts of the globe via air travel, killing nearly 800 people. As the H5N1 virus is detected in poultry in more countries, he said it's reasonable additional infections could occur along natural bird migratory routes in Europe, the Middle East and Africa. "I think during the last two days we have seen the citizens of Europe wake up to the reality that there is an epidemic of avian influenza in the world at this time," he said. "They have been confronted by television pictures of chickens and other birds being culled in countries that are very close to the European Union."