Most conventional surveillance systems are unable to identify emerging infectious diseases, which by definition are unexpected and may be caused by unusual or previously unknown pathogens.2 Timeliness is also often an issue, especially for rapidly moving outbreaks such as SARS or pandemic influenza. In recent years, approaches have been developed to circumvent these limitations of traditional surveillance. Outbreaks or other occurrences of concern are often referred to as ‘‘health events.’’ Considerable interest in recent years has focused on the detection of health events, rather than specific diseases, as a pathway to more generic an d timely surveillance. Some auth ors now make a distinction between ‘‘event -based’’ and tradition al (or ‘‘indicator-based’’) surveillance.26,27 The revised IHR document also emphasizes event-based reporting.23 Event-based surveillance can be viewed as a way of improving sensitivity and timeliness, with the goal of near real-time detection. In the past decade or 2, several very important developments have helped to move this goal closer to reality. These include ‘‘syndromic surveillance’’ (emphasizing the use of nontraditional and automated data sources), the evolution of ‘‘digital surveillance’’ using the internet and other computer-based systems, and the development of new enabling technologies in communications and diagnostics. These categories are not mutually
exclusive, and there is considerable overlap.