The world could at any time be faced with a massive flu outbreak like those in 1918 or 1968 that killed tens of millions of people, the World Health Organization warned, urging countries to be prepared. "History has told us that no one can stop a pandemic. The question is: when is it going to happen?" WHO spokeswoman Margaret Chan told reporters. "I don't think anybody has the answer to it. We have to be on the lookout for any time, any day," she added. Deadly avian influenza, which has killed 55 people in Asia since resurfacing in 2003, has...