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.
The researchers noticed that the bills’ move according to two mathematical rules, each known as a power law. One describes the distance travelled in each step of the journey, the other the length of time spent between journeys.
While most notes travel a short distance each time, there is a slim probability that it will leap a very long distance – perhaps carried from one side of the US to the other in the wallet of a passenger taking a flight. Secondly, while some notes move on quickly, there is a fair chance that it will remain in one place for a long period – for instance stuffed into a child’s piggy bank.
Although the movements of individual bills remain unpredictable, the mathematical rules make it possible to calculate the probability that a bill will have travelled a certain distance over a certain amount of time. “What’s triggering this is our behaviour,” Brockmann told New Scientist. “That is what you need if you want to build quantitative models for the spread of disease.”
Very, very important
Brockmann admits that the movement of money may not perfectly mirror that of people. For one thing, he says, it may be that only certain types of people are interested in seeing where their bills have been and entering that on www.wheresgeorge.com. However, he says comparing the model to publicly available information on passenger flights and road travel suggests that it is accurate.
Luis Amaral at Northwestern University, US, believes the study could indeed prove very useful to epidemiologists. “Understanding the way people move can be very, very important for developing strategies for fighting disease,” he told New Scientist. “It seems like a very cool study.”
But Amaral also says that the comparison between banknotes and disease is far from perfect. “Banknotes do not reproduce like a disease,” he notes.