Seasonal influenza (the flu) is an acute respiratory infection caused by influenza viruses . It is common in all parts of the world. Most people recover without treatment. Influenza spreads easily between people when they cough or sneeze. Vaccination is the best way to prevent the disease. Influenza refers to illness caused by the influenza viruses , but the term is commonly and incorrectly used to refer to similar illnesses caused by other viral respiratory pathogens. Influenza viruses are classified as type A, B, or C by their nucleoproteins and matrix proteins. Seasonal flu vaccines in the United States are formulated to protect against three influenza viruses known to cause human epidemics, including: one influenza A (H1N1) virus , one influenza A (H3N2) virus, and one influenza B/Victoria lineage virus. Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses . Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms begin one to four (typically two) days after exposure to the virus and last for about two to eight days.
