To address these drawbacks, a complementary system of informal surveillance tools have been developed, some by governmental agencies, but many by non-governmental organizations and/or researchers. Event-based surveillance systems such as ProMED, GPHIN HealthMap and BioCaster rely on unofficial reports of disease, for example from clinicians or web-based healt-related news media, to report on disease outbreaks [15] [16] [17] . Such systems have proven reliable and timely and informal sources of information were even recognized in the 2005 revision of the International Health Regulations as important sources of epidemic intelligence [18, 19] . The rapid and accelerating growth of the Internet has improved the usefulness and sensitivity of these systems and they have likely improved the timeliness of outbreak reporting [18] , and the ever-expanding availability of electronic information has also led to the discovery of other types of analyses that detect disease outbreaks. ''Web-crawlers'' (software programs that search internet sites for specific terms, and then use these search terms to generate reports or maps of disease activity) can provide important information on disease outbreaks that may be published on nongovernmental websites, in online newspapers, and in blogs, and this approach powers the widely-used HealthMap system mentioned above [20] . In the context of the recent cholera outbreak in Haiti, there were inconsistencies in initial accounts of regional disease activity, but information from HealthMap proved useful in the construction of a mathematical model that predicted disease spread on the island [21] .