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 any 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." said Fukuda. "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."
John Oxford, professor of virology at St. Bart's and Royal London Hospital, agreed.
"I don't think the 2 billion figure should scare people because it's not as though 2 billion people are going to die. The prediction from WHO is that 2 billion people might catch it. Half of those people won't show any symptoms. Or if they show any symptoms, they will be so mild they will hardly know they've had it."
Fukuda said it also is impossible to say if the current strain of swine flu will become severe or mild, but that even with a mild flu, "from the global perspective there are still very large numbers of people who could develop pneumonia, require respirators, who could die."
A mild outbreak in wealthier countries can be "quite severe in its impact in the developing world," Fukuda said.
People react differently to the flu depending on their general state of health and other factors, he said. Some younger people in the Southern Hemisphere may be more vulnerable because of poor diet, war, HIV infections and other factors.
"We expect this kind of event to unfold over weeks and months," Fukuda said. "Really if you look over a two-year period that is really the period in which you see an increase in the number of illnesses and deaths during a pandemic influenza."
So far the swine flu virus has spread to 26 countries. Brazil and Argentina on Thursday became the second and third countries in South America to announce confirmed cases.
Mexican dance halls, movie theaters and bars were allowed to fully reopen Thursday after a five-day shutdown designed to curb the virus' spread. Businesses must screen for any sick customers, and restaurant employees must wear surgical masks.
Fans can attend professional soccer matches this weekend after all were played in empty stadiums last weekend.
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.
This swine flu seems to have a long incubation period five to seven days before people notice symptoms, according to Dr. Marc-Alain Widdowson, a medical epidemiologist from the U.S. Centers for Disease Control and Prevention now tracking the flu in Mexico City. That means the virus can keep being spread by people who won't know to stay home.
Laughing and joking, high school students gathered at the entrance of the National School of Graphic Arts in Mexico City, waiting to fill out forms that asked about their health.
Of 280 students entering the school in the first 20 minutes, two showed symptoms of swine flu, including coughing and nasal congestion, said assistant principal Ana Maria Calvo Vega. Their parents were notified and they won't be readmitted without a statement from a doctor saying they don't have the virus, she said.
Students at a Mexico City vocational high school were welcomed with a hand sanitizer and a surgical mask. Joyful to see each other again, students embraced and kissed some through masks.
But some worried that the virus could surge back once young people gather in groups again.
"My 17-year-old daughter is afraid. She knows she must go back but doesn't want to," said Silvia Mendez as she walked with her 4-year-old son, Enrique, in San Miguel Topilejo, a town perched in forested mountains near the capital.
Working parents have struggled to provide child care during the shutdown. It forced many to stay home from work, bring their youngsters to their jobs, or leave them at home.
Each school, Mexican officials said, had to be cleaned and inspected this week. Complicating the task: Many schools are primitive buildings with dirt floors and lack proper bathrooms. It was unclear how students attending those schools could adhere to the government's strict sanitary conditions.
The government promised detergent, chlorine, trash bags, anti-bacterial soap or antiseptic gel and face masks to state governments for delivery to public schools. But some local districts apparently didn't get the word.
U.S. health officials are no longer recommending that schools close because of suspected swine flu cases since the virus has turned out to be milder than initially feared. But many U.S. schools have done so anyway, including the school of a Texas teacher who died.
In Asia, top health officials said the region must remain vigilant over the threat of swine flu, stepping up cooperation to produce vaccines and bolstering meager anti-viral stockpiles.
The virus has so far largely spared Asia. Only South Korea and Hong Kong have confirmed cases. On Thursday, China and Hong Kong released dozens of people quarantined over suspected contact with one of the region's few swine flu carriers.
Experience has been the spur to WHO to make sure the world is as prepared as possible for a pandemic, which would be indicated by a rise to phase 6 from the current phase 5 in the agency's alert scale. That would mean general spread of the disease in another region beyond North America, where the outbreak so far has been heaviest.
"I'm not quite sure we know if we're going to phase six or not or when we would do so," Fukuda said. "It's really impossible for anybody to predict right now."
Officials said the agency was likely to shorten its annual meeting of its 193 member states later this month from 10 days to five because of the outbreak, which it was scheduled to discuss.
"That is under consideration," Fukuda said. "Sure it is possible."