Northwestern researchers base their model of the spread of H1N1 on estimates of the rate at which money flows through the economy.

At the heart of his simulation are two immense sets of data: air traffic and commuter traffic patterns for the entire country, and the yield of a whimsical Web site, Where’s George?

Where’s George? was started more than 10 years ago by Hank Eskin, a programmer who marked each dollar bill he received with a note asking its next owner to enter its serial number and a ZIP code into the Web site, just for the fun of seeing how far and fast bills traveled. By 2006, the site had the histories of 100 million bills.