A few days ago, someone from the European Central Bank emailed Dirk Brockmann to ask for advice on how contaminated Euro notes might influence the spread of bird flu. Brockmann, who recounted this story in his invited talk today, explained that they’d made a common mistake in misunderstanding his work.

Back in January, Nature published an article by Brockmann and his colleagues that looked at how dollar bills travel across the United States. The paper is here. The idea was to study the dollar bills’ movements as a proxy for how people travel. A model based on the dollar bills’ diffusion pattern could then be used in epidemiology, to help predict how diseases spread. But he was not, he emphasised, suggesting that bank notes themselves spread disease.

There’s another interesting aspect to this story, too.

The data for the study came from Where’s George?! – a website that collects sightings of marked dollar bills from enthusiasts across the United States. If you register, you get a small rubber stamp that says “Track me at www.wheresgeorge.com”. You mark your bills, throw them back into circulation (ie, spend them) and then watch to see where they turn up. People that really get into this call themselves “Georgers” and meet up around the country.

A few turned up at today’s session, to hear Brockmann speak. Before I took my spare dollar bills back to Britain, I went to talk to them…

Karin Printz, a Georger who lives not far Baltimore, had heard about the game from her brother but only got sucked in herself after getting a marked bill in change at a restaurant. She registered her first notes last month, and got her first hits yesterday. “I was terribly excited.”

Next to her was a veteran of the game. Jim Malone, from Annapolis, had started playing in 1999. He said that he had personally registered around 7,000 bills and got “hits” (meaning new sightings) on 1,100 of them. “One got to Puerto Rico,” he said. But he said others have done more: the site’s record-holders have registered hundreds of thousands of bills and seen their cash go right round the world.

It’s not that they’re obsessive, he said. “We prefer extremely focussed.”

A special guest at the session was Hank Eskin, who had set up the Where’s George?! website. He’d had to struggle to keep the site online as news of Brockmann’s work appeared on news sites around the world last month. Right now, nearly 80 million bills are registered.

For the Georgers, having Brockmann use their data has been a blessing. A hobby that friends used to laugh at has suddenly become something useful.

Malone was looking forward to going out for dinner with Brockmann, to quiz the man about his work. He unfurled a copy of Brockmann’s Nature paper, which had been stashed in an inside coat pocket. “I’ve read the paper about five times now, and each time I pick up something different.”