"Scientists who track flu fear  new, powerful strains"

In this story:    Influenza has deadly history   Researchers track global movement

ATLANTA (CNN) -- As the United States and Europe battle yet another cold and flu season, scientists are having more trouble tracking and predicting where and when the next strain will pop up. As global travel becomes easier, the highly  contagious virus can now hitch a quick ride to any part of the world at any time of the year, disrupting what has been a predictable pattern. "We are very worried we will have a  worldwide pandemic of influenza that will affect probably 40 percent of the world's  population," said Dr. Steven Mostow of the University of Colorado. Mostow said the  number of people killed from such a wave of influenza could rival some of the worst  outbreaks in modern times.

Influenza has a deadly history In the late 19th century, more than 20 million  people worldwide died as a result of the flu. In 1917, flu outbreaks killed more people  than those who died during World War I. In 1968, a wave of the Hong Kong flu claimed some 700,000 lives. "We will never be able to control the flu like we control polio, like we control smallpox," said Mostow. "Because the flu virus isn't stable. It mutates. And it is smarter than we are." Researchers at the U.S. Centers for Disease Control and Prevention and the World Health Organization track movement and mutations of each flu outbreak as they travel around the globe. They do this to help them decide which strains to attack with each year's flu vaccine.

Researchers track global movement

Normally, the tracking of a strain begins in China, where scientists believe influenza outbreaks begin. Flu pops up in areas where humans and animals live in close contact, with birds passing an avian form of the flu to pigs.

Elements of that strain are eventually passed to people. The flu commonly moves from China east across Asia to Alaska, where it works its way south into the heart of North America. "It then mutates and moves through the U.S. over a several-week period," said Mostow, "jumps the Atlantic and goes into western Europe, England, France and Germany, Switzerland, et cetera."

In June, July and August, flu trackers monitor strains that are then usually

circulating in the Earth's Southern Hemisphere. "It jumps below the equator, showing up in Africa, South America, Australia and New Zealand," Mostow said.

A new parent strain of influenza has emerged about every 100 years,

according to experts. The current troublemaker, known as the Sydney virus, is a mutation of its parent strain, the notorious Hong Kong flu, which claimed so many victims 30 years ago. "The problem is," said the CDC's Dr. Nancy Cox, "we can't predict when it will occur. As one of my colleagues said, the influenza clock is ticking, but we don't know what time it is."

Health Correspondent Holly Firfer contributed to this report.