There is some evidence that influenza pandemics occur on a...

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.

"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.