Respiratory viruses are very active in the fall and winter. The viruses that can sometimes cause more serious illness are influenza (flu), respiratory syncytial virus (RSV) and SARS-CoV-2 (which causes COVID-19). These viruses spread quickly. When infected, most people will have a mild illness. But some people become seriously ill and can die. Learn more about the spread in your community and steps you can take to protect yourself.