I. Introduction
Influenza is one of the most threatening infectious diseases, causing approximately three to five million cases of severe illness and 290 000–650 000 deaths worldwide each year.1 To monitor influenza outbreaks, the U.S. and Europe operate their own influenza surveillance systems through the Centers for Disease Control and Prevention (CDC) and the European Influenza Surveillance Scheme, respectively, which provides weekly influenza-like illness (ILI) reports.
https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal)