> Seasonal influenza, or flu, is a highly contagious respiratory illness
caused by influenza viruses that infect the nose, throat and lungs. The
two main types of influenza viruses—types A and B—are the viruses that
typically spread in people (human influenza viruses) and cause annual flu
outbreaks. Each year, influenza causes millions of illnesses worldwide
and, in the United States, results in thousands of hospitalizations and
deaths. Influenza is especially dangerous for people 65 years and older,