Is flu on the rise among workers? Those working in public health track the number of flu-related hospital and doctor visits, but many people suffer symptoms and don’t seek medical treatment. So, how do we know how many people are sick with the flu during a flu pandemic or a seasonal epidemic? Each year, the Centers for Disease Control and Prevention (CDC) uses a mathematical model to estimate the total number of flu illnesses in the United States, but this is not done until the end of the flu season.