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Home >> Life
UPDATED: 08:45, January 27, 2006
Tracking dollar bills helps explain how diseases spread
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Even for scientists, it is an unusual use of half a million dollars. Instead of spending it, researchers tracked the movements of individual dollar bills as they changed hands across the United States, in an attempt to understand how future epidemics might sweep around the globe.

The study promises to shed light on one of the toughest problems infectious disease experts have to wrestle with: how does travel help spread infectious diseases?

"If you want to understand how disease spreads, you need to understand how humans travel, because a lot of the time humans carry the disease from one place to the next," said Dirk Brockmann at the Max Planck Institute for Dynamics and Self-Organization in Gottingen, Germany.

"It's clear people travel long distances less often, but make a lot of short trips, but what are the details? How often are trips of a certain distance made?"

To find out, the scientists turned to a US website called Set up as a game, the site lets people register dollar bills after marking them with the website's name. Other users around the world report when a marked dollar bill falls into their hands, noting where they were at the time.

The site tracks bills worth a total of more than US$50 million bills around the globe, but for the study Dr Brockmann looked at the movement of 500,000 notes that stayed within the United States. His team tracked the batch of bills, some over tens of thousands of kilometres.

Over a period of two weeks, many stayed in the state they were first tagged in, but a small portion left the state, while an even smaller percentage travelled thousands of kilometres.

While 52.7 per cent of bills marked in Seattle were still there two weeks later, 7.8 per cent had travelled more than 800 kilometres away. Similarly in New York, only 7 per cent of bills tagged travelled more than 800 kilometres in a fortnight.

The study allowed the scientists to draw up equations describing the probability of people travelling and so carrying diseases to new regions.

"What we can do now is, given a particular disease, if we know the speed it spreads from A to B, we can work out the time it'll take to get to other places," said Dr Brockmann.

Neil Ferguson, an expert in modelling infectious diseases at Imperial College London, said: "Although they're looking at the movement of money, that's indicative of how people move around. A few people travel a lot and if a bill gets into one of their wallets, then it can easily turn up on the other side of the country.

"That variability is very important in terms of modelling disease. The jet set are crucial when it comes to spreading infections, and being able to put numbers on the behaviour of those people is very important."

Source: China Daily

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