Up to 2 billion people could be infected by swine flu if the current outbreak turns into a pandemic lasting two years, the World Health Organization said Thursday. WHO flu chief Keiji Fukuda said the historical record of flu pandemics indicates one-third of the world's population gets infected in such outbreaks. Independent experts agreed that the estimate was possible. In Mexico, the hardest hit country so far, high schools and universities opened for the first time in two weeks as the government's top health official insisted the epidemic is on the decline. All students were checked for swine flu symptoms and some were sent home. "If we do move into a pandemic, then our expectation is that we will see a large number of people infected worldwide," Fukuda said. "If you look at past pandemics, it would be a reasonable estimate to say perhaps a third of the world's population would get infected with this virus."