The increasing number of emerging infectious disease events that have spread internationally, such as severe acute respiratory syndrome (SARS)

Free or low-cost sources of unstructured information, such as Internet news and online discussion sites, provide detailed local and near real-time data on disease outbreaks, even in countries that lack traditional public health surveillance. To improve public health surveillance and, ultimately, interventions, we examined 3 primary systems that process event-based outbreak information: Global Public Health Intelligence Network, HealthMap, and EpiSPIDER. Despite similarities among them, these systems are highly complementary because they monitor different data types, rely on varying levels of automation and human analysis, and distribute distinct information. Future development should focus on linking these systems more closely to public health practitioners in the fi eld and establishing collaborative networks for alert verifi cation and dissemination. Such development would further establish event-based monitoring as an invaluable public health resource that provides critical context and an alternative to traditional indicator-based outbreak reporting.

The widespread adoption of increasingly sophisticated forms of information technology has paralleled the increase in rapid and far-reaching international travel. The emergence and global spread of the 2009 pandemic influenza A (H1N1) virus illustrated not only the hazards of an interconnected world, but also the powerful role of new methods for detecting, tracking, and responding to infectious diseases. 1 Although formal reporting, surveillance, and response structures remain essential to protecting public health, 2 a new generation of freely accessible, online, and real-time informatics tools for disease tracking are expanding the ability of public health professionals to detect weak signals across borders and to raise earlier warnings of emerging disease threats. [3][4][5] THE HEALTHMAP H1N1 SYSTEM HealthMap (http://healthmap.org) is an example of a new effort in transparent, global, public health surveillance. 6,7 The HealthMap system combines automated, around-the-clock data collection and processing with expert review and analysis to aggregate reports according to type of disease and geographic location. HealthMap sifts through large volumes of information on events, obtained from a broad range of online sources in multiple languages, to provide a comprehensive view of ongoing global disease activity through a freely available Web site.To enhance the situational awareness of public health professionals, clinicians, and the general public regarding the global spread of 2009 H1N1 influenza infection, HealthMap partnered with the Journal to create the H1N1 interactive map (available at www.healthmap.org/nejm) as part of the Journal's H1N1 Influenza Center (http://h1n1.nejm.org). This interactive map used the Health-Map infrastructure to provide information about outbreaks. The information was obtained from both informal sources The worldwide spread of the H1N1 virus is shown in Figure 1 and the interactive map available with the full text of this article at NEJM.org. The geographic location was recorded for all reports entered into the HealthMap system, allowing for easy tracking of the regional and global proliferation of H1N1 influenza. HealthMap also identified the increasing number of countries over time with informal reports of suspected or confirmed cases, according to WHO regions and pandemic phases ( Fig. 2 and the interactive graphic available at NEJM.org). By the end of WHO pandemic phase 4 (April 28), there had already been reports of either suspected or confirmed cases in 32 countries. During the initial phases of the pandemic, the spread of the virus to new countries was most rapid in Europe and the Americas. This spread gradually slowed over time to about one country per day through the end of phase 5 (June 10), and it further diminished to one country per 2 days in phase 6, possibly because of the implementation of containment measures and the early involvement of countries with a high volume of air travel. Although air travel may have driven the intercontinental spread of the virus, oth...