[INTERACTIVE: Interactive guides to spread of swine flu]
WHO flu chief Keiji Fukuda says the number wasn't a prediction, but that past experience with flu pandemics indicated one-third of the world's population gets infected.
Fukuda says that with a world population of 6 billion people, it's "reasonable" to expect that kind of infection tally.
He said WHO is unable to know what the future holds, and it is impossible now to say whether the pandemic would be mild or severe.
WHO has said it believes a global swine flu outbreak is imminent, and last week it raised its alert to five, one step short of a pandemic.
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