Economics bashers who claim it isn't a real science like biology, will be distressed to hear that money may be the root of all epidemics. (Though, shouldn't they be using Canadian currency?)
Tracking the movements of hundreds of thousands of banknotes across the US could provide scientists with a vital new tool to help combat the spread of deadly infectious diseases like bird flu.
Modern transport has transformed the speed at which epidemics can spread, enabling disease to rip through populations and leap across continents at frightening speed.
However, scientists possess few mathematical models to help them understand these movements and how this might govern the global spread of disease. To a large degree, this is because tracking the movements of so many people over such a large area is next to impossible.
But now physicists from the Max Planck Institute in Göttingen, Germany, and the University of Santa Barbara, California, US, have developed a model to explain these movements, based on the tracked movements of US banknotes.
Dirk Brockmann and colleagues used an online project called www.wheresgeorge.com (George Washington's image is on the $1 bill) to track the movements of dollar bills by serial number. Visitors to the site enter the serial number of banknotes in their possession and can see where else the note may have been.
The team tracked 464,670 dollar bills across the US using 1,033,095 individual reports. The fact the notes are carried by people suggests it is a good way of modelling other things that people may carry, including disease.