A web-game that encourages people to track the serial numbers of dollar bills as they move around the country has produced the best model to date for explaining some of the ways that infections diseases spread. Where's George? players mark their bills with WHERESGEORGE.COM; visitors to the site are encouraged to enter the serial number of the bill they've found and where they got it. In this way, the passage of a dollar-bill (or some other piece of infection) can be tracked around the country.
Researchers at the Max Planck Institute used the data-set from Where's George? to compile a highly accurate model of human travel, which, in turn, has been key to understanding the way that diseases spread:
The physicists were intrigued: Like viruses, money is transported by people from place to place. They found that the human movements follow what are known as universal scaling laws (from local to regional to long-distance scales). Using the game data, they developed a powerful mathematical theory that describes the observed movements of travelers amazingly well over distances from just a few kilometers to a few thousand. The study represents a major breakthrough for the mathematical modeling of the spread of epidemics.
(via Dan Gillmor)
Update: Amara sez, "a similar effort has been underway since 2002 in Europe with the introduction of the Euro. The Euro coin has two sides, one side which shows the same design in every Eurozone country, while the other side shows a design specific to the European country that minted it. Therefore, from 2002, scientists have had a natural laboratory to follow the movements of people because the introduction of that coin (location and time) in this process is precisely known. The Euro diffusion process can therefore show how epidemics spread and can show to what extent are Europeans integrating, and what are their travel patterns. A successful study depends on many people emptying their pockets and recording what is there, however, this link provides such a means to record your pockets.
Update 2: Ryan sez, "they actually only track Euro *notes*, not coins. Also, WheresGeorge.com has a brother Canadian site, called WheresWilly.com (after Wilfred Laurier, the man on the $5)."