evidence: Influenza, also known as the flu, is a highly contagious respiratory infection caused by flu viruses. The respiratory system is the parts of your body that help you breath. Seasonal flu is caused by viruses that already circulate among people. While seasonal flu viruses are detected year-round, in the United States flu season usually starts in late fall and lasts throughout the winter, sometimes into early spring. Most people have some immunity and a vaccine is available to help prevent the flu.