The World Health Organization says the H1N1 swine-flu virus could infect up to two billion people over the next two years - about one of every three people in the world.
A senior official for the U.N. agency, Keiji Fukuda, told an Associated Press interviewer Friday that WHO has been reporting only laboratory-confirmed cases thus far, but those are really only a fraction of the total number of swine-flu infections.
Speaking in Geneva, the health agency's headquarters, Fukuda said the global epidemic is still in its early stages.
A separate WHO report Friday said the virus has spread to almost every country in the world, killing about 800 people since it emerged in April.
In the United States, the Centers for Disease Control and Prevention estimates that up to 40 percent of Americans could contract swine-flu over the next two years. Unless an effective swine-flu vaccine is developed and produced successfully, the American government's experts say up to several hundred thousand people could die nationwide.
The first U.S. human trials of a potential swine-flu vaccine could begin in a few weeks. If all goes well, Fukuda said the first vaccine doses could be available before cold weather - the normal peak season for flu - arrives in the Northern Hemisphere in a few months.
The White House said Friday that it has been holding regular meetings to prepare for an expected spike in U.S. infections after September.
The U.S. has the highest number of swine-flu fatalities of any country in the world, followed by Argentina and Mexico. In Britain, Europe's hardest-hit country, new cases of swine flu spiked last week to 100,000, nearly twice as many as the week before.
Several pharmaceutical companies have indicated they will give out vaccines to developing countries at a reduced cost when they become available.
WHO spokesman Gregory Hartl said that after developing countries vaccinate health care workers, the organization will give them the choice of whom to treat. He said young people and adolescents are still the most likely population groups to become infected with the virus, but that the more severe cases seem to appear among older people.
WHO declared in June that the spread of swine flu worldwide had reached a pandemic stage, where large-scale outbreaks could be expected in multiple locations.
Some information for this report was provided by AFP and AP.