SEATTLE — Weeks before the Centers for Disease Control and Prevention and the World Health Organization alerted the public to a growing number of swine flu cases, a startup based in Seattle's suburbs had a hunch something was up.

Veratect Corp., a 2-year-old company with fewer than 50 employees, combines computer algorithms with human analysts to monitor online and off-line sources for hints of disease outbreaks and civil unrest worldwide. It tracks thousands of "events" each month — an odd case of respiratory illness, or a run on over-the-counter medicines, for example — then ranks them for severity and posts them on a subscription-only Web portal for clients who want early warnings.

The idea is that blogs, online chat rooms, Twitter feeds and news media and government Web sites are full of data that could indicate problems like swine flu.

Veratect attracted attention in recent days by publicly posting a timeline of the outbreak and publishing short reports on Twitter, where more than 4,000 people signed up to receive updates.

But skeptics question whether these companies can reliably detect meaningful signals from all the noise online or whether they are mainly good at spotting patterns in hindsight.

Veratect says it posted a report to clients April 6 describing an unusual number of respiratory illnesses in the Mexican state of Veracruz. Its computer systems troll the Web for reports that seem out of the ordinary, and a team of about 30 analysts, many of them multilingual holders of public health degrees, chase down the ones that seemed most alarming.

A key clue came in Mexican media reports on April 6 that in a Veracruz community called La Gloria — now considered a swine flu hot spot — local residents were blaming waste from a nearby pig breeding farm for respiratory illness, while health officials pinned it on a fly.

"Playing the blame game is one of those indicators" that something unusual is going on, said James Wilson, Veratect's chief scientist.

But some public health experts say it's not possible to draw firm conclusions from such data reports. "They are considered interesting, unofficial, instructive, imaginative, and then I would go back and emphasize unofficial," said Dr. William Schaffner, a public health expert at Vanderbilt University.

Even with the flaws, clients like World Vision, a large Christian humanitarian organization, pay Veratect for its tips. Recently, World Vision shifted resources — water purification tablets and education staffers — to areas Veratect thinks might see cholera outbreaks, said an executive.

A Veratect rival, iJet Intelligent Risk Systems, said it monitors emerging health risks, civil unrest and issues such as telecommunications outages, and counts the CDC among its paying clients.

Other efforts focus more narrowly. ProMed, a system designed by the Federation of American Scientists, lets human, animal and plant specialists share infectious disease information. A site called HealthMap compiles data from ProMed, the CDC, the World Health Organization and other sources. FluWiki has tracked bird flu since 2005, and last year Google launched Flu Trends.