By following the money trail, researchers may be able to predict an epidemic's itinerary.
A German-led study used data from a cash-tracking website to trace more than $450,000 worth of $1 bills through the continental United States, allowing scientists to develop a model of human dispersal that may prove crucial in fights against flu or other contagious diseases.
"The aim of our study was to be able to attach a number to human traveling behavior," said Dirk Brockmann, a theoretical physicist at the Max Planck Institute for Dynamics and Self-Organization in Goettingen, Germany. "If you want to understand how modern epidemics spread, you need to make some assumptions about how humans spread."
The researchers, from the Max Planck Institute and UC Santa Barbara, used data from www.wheresgeorge.com as a proxy for how people move.
The website allows users to specify where they found a marked bill, with each set of entries yielding a unique travel diary. One bill, for example, passed through eight states, assorted restaurants, a racetrack and a strip club on its journey of at least 4,191 miles.
The study, published Thursday in the journal Nature, scrutinized the first and second recorded locations of 464,670 dollar bills. Most reappeared within six miles of their origins, although a small but significant number journeyed 500 miles or more.
Analyzing the bills' paths allowed the researchers to create a mathematical model of the movement of people.
Brockmann said the model breaks down if population densities become too low or travel options too limited. Still, he believes simple but accurate assumptions may be key to realistic models of how a disease may spread -- and how it may be contained with intervention.