GENEVA — 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 yesterday.

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 but pointed out that many would not show symptoms.

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."

With the current total population of more than 6 billion, that would mean an infection total of 2 billion, he said, but added that the world has changed since pandemics of earlier generations, and experts are unable to predict if the impact will be greater or smaller.

"We don't really know," Fukuda said. "This is a benchmark from the past. Please do not interpret this as a prediction for the future."

Chris Smith, at flu virologist at Cambridge University in England, said the 2 billion estimate was possible.

"That doesn't sound too outlandish to me for the simple reason that this is a very infectious virus," Smith told The Associated Press. "You're talking about a virus that no one in the population has seen before and therefore everyone is immunologically vulnerable. Therefore it's highly likely that once it starts to spread, people will catch it. And since the majority of the world's population are in contact with one another, you're going to get quite a lot of spread."

So far the swine flu virus has spread to 26 countries. Brazil and Argentina yesterday became the second and third countries in South America to announce confirmed cases.

A Japanese boy in Chicago has tested positive for swine flu, Japanese Foreign Minister Hirofumi Nakasone said.

The foreign ministry was informed of the case through the boy's family via consular officials in the U.S., Nakasone told reporters in Tokyo yesterday. The 6-year-old boy is recovering, public broadcaster NHK said.

There have been no confirmed cases of swine flu in Japan.

Mexico confirmed two more deaths, for a total of 44, while 1,160 people have been sickened, up 90 from Wednesday. Despite death tolls and confirmed caseloads that rise daily, Health Secretary Jose Angel Cordova insisted the epidemic is waning in Mexico.

WHO raised its global total of laboratory-confirmed cases to 2,099, from 1,893 late Wednesday, and said swine flu also has caused two deaths in the United States.