"It started as a side project," Freifeld says of HealthMap. "It's really kind of taken off in a huge way. It's been kind of surprising."

Funded by Google.org, Outbreaks Near Me has already been downloaded more than 1,500 times. "We're definitely just beginning to start this process," says Brownstein, adding that he hasn't had a chance to go through all of the tips already submitted by users. "There were a few interesting reports about school closures and illnesses at university," he says.

The HealthMap team is also working on versions of the software for other mobile platforms, including BlackBerry devices and those running the Google Android operating system.

A simple text-messaging system to alert those with basic mobile phones of outbreak information is also on the agenda. That could prove particularly useful in the developing world, where smartphones are not yet common and information about infectious diseases is sometimes hard to find.