Ideally, flu vaccine or flu shot is distributed annually within and outside the United States. Getting vaccinated is vital to help protect people from circulating strains. When Flu Season Starts: Causes and Symptoms of Flu Flu season typically begins in the fall and peaks during winter. The flu, or influenza, is a highly contagious respiratory illness caused by influenza viruses. These viruses can be categorized into different types (A, B, and C), with subtypes and ...