This web page provides weekly, preliminary estimates of the cumulative in-season numbers of flu illnesses, medical visits, hospitalizations, and deaths in the United States. CDC does not know the exact number of people who have been sick and affected by influenza because influenza is not a reportable disease in most areas of the U.S. However, CDC has estimated the burden of flu since 2010 using a mathematical model that is based on data collected through the U.S. Influenza Surveillance System, a network that covers approximately 8.5% of the U.S. population (~27 million people).