Scientists think they may have found a better way to predict how diseases like a global flu epidemic could spread: Follow the money.

Using the popular "Where's George?" Web site that tracks U.S. dollars, researchers developed a mathematical tool that could help chart the path of an infectious disease.

"We are optimistic that this will drastically improve predictions about the geographical spread of epidemics," said Theo Geisel of the Max Planck Institute in Germany, which developed the tool along with the University of California, Santa Barbara.

Details appear in Thursday's issue of the journal Nature.

Fears of a global flu epidemic have arisen from the spread of bird flu, a virus that has killed more than 70 people in Asia and Europe since 2003, but which is so far spreading easily only among poultry. International health officials fear the virus could mutate into a form easily transmitted between humans.

So far, the bird flu virus is not easy to catch. But experts have warned that if it ultimately begins spreading among people, travelers are the most likely way it will become a worldwide threat.

Tracking travelers is difficult, so researchers came up with the idea of studying them indirectly by tracing how money circulates through the economy.

In the study, scientists traced the whereabouts of nearly half a million dollar bills on http://www.wheresgeorge.com bill-tracking site.

Users register their money and then spend it. They can monitor the money's movement online as it changes hands.

Researchers found that most of the money (57 percent) traveled between 30 miles and 500 miles over about nine months in the United States. About a quarter of the bills moved more than 500 miles.

By analyzing the movement of money — and human travel — over different distances, the scientists found that the money followed a predictable pattern.

The method could be used to create more realistic disease models that track the spread of germs and perhaps prevent outbreaks, they say.

The study is the most detailed to date showing the variability of travelers, said Neil Ferguson, a professor of mathematical biology at Imperial College in London.

"From the perspective of disease modeling, one thing we would like to understand better is the variability between people in their traveling," said Ferguson, who had no role in the research.