...And I'll show you how diseases may travel.

george.jpg Credit: Max Planck Institute for Dynamics and Self-Organization.

“See where I’ve been. Track where I go next. Please go to www.wheresgeorge.com and enter the serial number and series.”

Thousands of people have come across a variation of this statement stamped in blue, black or red ink on a US banknote, and have obeyed the printed instructions and gone to wheresgeorge.com to follow the trail of their cash. To date, more than 76-million bills have been entered into the wheresgeorge.com database. But the website and its tracking abilities are more than a diversion. According to a study published in the January 26th issue of Nature, it may provide important information that will help us understand the next global pandemic.

Physicist Dirk Brockmann and his colleagues from the Max Planck Institute for Dynamics and Self Organization in Göttingen, Germany hypothesized that, since currency is carried by people as they go from place to place, currency movement mirrors human movement. Using the vast storehouse of data collected at wheresgeorge.com, the researchers are studying human travel in an attempt to improve models of disease transmission.

“If you take 10,000 bills that start in New York or LA, you can see how they distribute in the country,” Brockman said. “That gives me a proxy for estimating how likely a human is to travel a distance of 100 miles as compared to say a distance of 10 miles or 1,000 miles.”

“That is essential information that you need if you want to make quantitative models for the spread of disease.”

An epidemic spreads as infected individuals move around the world. The 14th-century plague that ravaged Europe took three years to move from south to north across the continent. At that time, humans travelled no farther in a day than a horse could carry them; contemporary humans, however, are far more mobile.

Brockmann and colleagues first hypothesized that a person’s degree of mobility is dependent upon where they live in the US. Surprisingly, the data from wheresgeorge.com did not fit that hypothesis. Patterns of motion were similar across the country. Money moved in the same fashion in the Midwest as it did on both coasts, and in both urban and rural environments.

“We saw that apparently human travel—despite the fact that humans travel by flight and by car and by train and all that—is governed by some very simple underlying rules,” Brockmann said. “These rules are independent of what kind of region you look at.”

The researchers did not find an individual value to represent the average distance of travel, but devised a probability function to model it. Obviously, not all people traveled the same distance daily, but the probability that a person would travel 10 miles in a day or 100 hundred miles was universal.

While the pattern of human travel observed in the US would probably not be the same as the pattern in Africa or Antarctica, Brockmann expects that the patterns will be similar in Europe and Asia.

Brockmann hopes that this study will cause people to re-evaluate old models of disease transmission, particularly those that use a single number to represent the average distance of human travel.

“There are thousands of models and now we can look at which ones are the ones that should be developed further and which ones are the ones that should be thrown in the bin.”

Originally published February 6, 2006

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