Influenza, commonly called "the flu" is caused by the influenza virus, which infects the respiratory tract (nose, throat, and lungs). Influenza is a leading cause of illness in the United States and can lead to serious medical conditions, hospitalization, or even death. Flu seasons are unpredictable. They can begin early in the fall and last late into the spring. Influenza is extremely contagious and spreads very easily in communities. The best way to protect yourself from the flu is by getting an annual flu vaccine as soon as it becomes available.