Healthmap is one of a number of new programs springing up to track infectious disease, using unconventional sources like emergency-room admittance, over-the-counter prescription sales, and school absenteeism reports. In 2008, Google found that flu season in a given location correlated with certain keywords that users typed into its search engine. Google launched an application called Google Flu Trends, and published a paper in Nature, reporting that the system could detect an outbreak of influenza two weeks before the CDC issued a confirmation. The CDC method, which includes molecular testing to confirm specific viruses, is more accurate but also time- and resource-intensive.

Internet-based approaches may prove especially useful in tracking new pathogens. "New diseases can be the most devastating because the human body and the public health infrastructure are not set up to deal with it," says Marc Levy, deputy director for the Center for International Earth Science Information Network at Columbia University. "That's why this new technology is so important, because it dramatically simplifies the process of detecting new diseases in a geographically accurate way."

Levy cautions that as new technologies like HealthMap become more common, they may suffer from a Big Brother bias--governments that hold a tight rein may choose to shut down access to news outlets or Internet sites, creating an information blackout. "There's a danger that these powerful tools would be the target of restrictions," says Levy. "But those are risks worth tolerating."

To create HealthMap, Brownstein and his colleague Clark Freifeld, a research software developer at Children's Hospital Informatics Program, devised algorithms to sift through various Internet sources and news aggregators like Google, looking for keywords in seven different languages, and gauging a source's reliability. Verified reports are flagged to a corresponding location on an electronic map of the world, allowing users to watch a disease like H1N1 unfold by clicking through a timeline below the map. Users can also click on a flag, which links to a news report, hospital press release, or blog entry.

In the future, Brownstein and Freifeld plan to tap into the minute-by-minute world of social networking, and will monitor sites like Twitter and Facebook for even earlier signs of disease outbreaks. "We're walking a fine line getting into these data sources," says Brownstein. "Twitter is a very difficult source, with a very short amount of text that's hard to verify. One individual case might not tell us something is happening, but if we're having enough people reporting in a location, that might mean something. So we'll wait for a threshold of reporting, instead of relying on just one tweet of, say, an Ebola outbreak."