About half of people who use the Internet to access health information online do so via a search engine; thus, monitoring the queries received by search engines is a potential biosurveillance strategy. A rapid increase in the number of Google searches containing the word "fever'' would be of concern in the absence of a known outbreak or other explanation. In contrast to website monitoring, monitoring of queries to the three most popular search engines would catch nearly 80% of the health-related searches issued over the Internet, assuming that people do not switch to less commonly used search engines for health-related searches.
Privacy policies of the search engines (and websites) are, however, a barrier to developing a system to monitor query data from search engines. Organizations that operate search engines (and websites) respect the rights of individuals to confidentiality and have strict policies concerning the distribution and use of personal information. There are "privacy-protecting'' techniques that might allow society to benefit from this information (should research prove its value), while protecting an individual's right to confidentiality. For example, an organization operating a search engine could run biosurveillance algorithms within its computing facility. It could send only the results (e.g., daily or hourly counts of queries that matched predefined criteria from a city or county) to governmental public health.
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