There is some evidence that influenza pandemics occur on a regular cycle, with one every 20 to 30 years. The notorious Spanish flu killed at least 20 million people worldwide in 1918. Outbreaks in the late 1950s and late 1960s resulted in tens of thousands of deaths in the United States alone.
Now the world may be due for another outbreak.
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"Almost everyone in the field feels that an influenza pandemic is virtually inevitable, and that we need to be prepared for it," said Stephen Morse, the director of the Center for Public Health Preparedness at Columbia University in New York.
The World Health Organization estimates the H5N1 virus could infect up to 30 percent of the world's population. Shigeru Omi, the WHO official who issued last month's warning, said that estimates of 2-7 million deaths were "conservative" and that the maximum range could go as high as 50 million deaths.
Some virologists, however, take issue with his warning.
"This alarmist warning is irresponsible in using this language to rouse the public's fear," said Michael Lai, a virologist at the University of Southern California in Los Angeles.
He says the warnings of an impending flu pandemic have circulated since the Hong Kong outbreak in 1997, but no major outbreak among humans has yet happened.
Furthermore no one actually knows what would happen to the virus if it adapted to grow in humans. It might be as lethal as it is now, or it might become far less pathogenic.
Since palliative care, or disease treatment, is much better today than in 1918, deaths are likely to be far fewer, some health experts argue.
Still, any pandemic is likely to spring from developing countries with limited means to fight it.
"There is at present no effective way to control the poultry endemicity for southeast Asia due to the lack of resources or political conviction," said Kwok-yung Yuen at the Center of Infection at the University of Hong Kong. "Only Hong Kong has [successfully maintained] bio-security in farms and markets."
The strong warnings, therefore, are mainly directed at governments rather than the general public, says Dick Thompson, a WHO spokesperson in Geneva, Switzerland.
"What we are trying to do is alert governments that steps need to be taken to prepare for the next pandemic," he said.
If that means warning of worst-case scenarios, so be it.
"Are we scaring people? I don't know," he said. "But rather than springing on people some terrible event, it's better that they get emotionally ready for what they could face. We think a pandemic is coming. Nobody knows when. But it is good to get people prepared before it arrives."
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