tool which based on Information Retrieval and Statistical Analysis methods uses the content of Twitter to nowcast flu rates in the UK. Digital methods Digital surveillance of public health largely relies on three methods: search-based trends on sites like Google and Wikipedia, social media posts on platforms like Facebook and Twitter, and participatory surveillance websites such as Flu Near You and Influenzanet. Search trends provide indirect data on public health, while the latter two methods provide direct data. Search aggregates Search aggregates have been most frequently used to track and model influenza. A popular example is Google Flu Trends, which was first released in 2008. Wikipedia has also been used, though it is potentially prone to "noise", as it is a popular source of health information whether a user is ill or not. During the COVID-19 pandemic a new methodology has been developed to model COVID-19 prevalence based on web search activity. This methodology has also been used by Public Health England in the United Kingdom as one of their syndromic surveillance endpoints. Social media Examples of social media public health surveillance include HealthTweets, which gathers data from Twitter. Twitter data is considered highly useful for public health research, as its data policies allow public access to 1% samples of raw tweets. Tweets can also be geolocated, which can be used to model the spread of contagious disease. It is the most used social media platform for public health surveillance. During the COVID-19 pandemic, Facebook used aggregated, anonymized data collected from its platforms to provide human movement information to disease models. It also offered users a chance to participate in a disease symptom survey through Carnegie Mellon University. Surveillance sites Flu Near You and Influenzanet are two examples of crowd-sourced digital surveillance systems. Both sites recruit users to participate in surveys about influenza symptoms. Influenzanet was established in 2009, and operates in ten countries in Europe. Its predecessor was Grote Griepmeting, which was a Dutch/Belgian platform launched in 2003 and 2004. Flu Near You is used in the US. Another example of a surveillance sites is Dengue na Web, used to survey for dengue fever in Bahia, Brazil. Laboratory-based surveillance Some conditions, especially chronic diseases such as diabetes mellitus, are supposed to be routinely managed with frequent laboratory measurements. Since many laboratory results, at least in Europe and the US, are automatically processed by computerized laboratory information systems, the results