Up to two billion people could be infected by swine flu if the current outbreak turns into a pandemic lasting two years, the World Health Organisation (Who) has said.
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.
But he added: "This is a benchmark from the past. Please do not interpret this as a prediction for the future."
Mr 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 six billion, that would mean an infection total of two billion, he said.
But he added 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 Mr Fukuda. "This is a benchmark from the past. Please do not interpret this as a prediction for the future."
Chris Smith, a flu virologist at Cambridge University, said the two billion estimate was possible.
"That doesn't sound too outlandish to me for the simple reason that this is a very infectious virus," he said. "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."
