Influenza, commonly known as "flu," is a respiratory disorder caused by an influenza virus. It is an infectious disorder which is of three types: type A, type B, and type C. The common symptoms of the disorder are cough, cold, fever, and sore throat, which can be mild or severe. The disease is spread easily from person to person by sneezing, coughing, or touching surfaces, and is generally diagnosed on the basis of symptoms. Seasonal outbreak of influenza primarily occurs in winter. Most people generally recover fully, but severe complications such as pneumonia can also develop. The treatment for influenza aims to prevent such complications and to ease the symptoms until the infection is cured. Initiation of antiviral treatment is recommended for hospitalized patients as early as possible. Annual vaccination for influenza is the best way to prevent its infection, as vaccination can be done well before the exposure to the influenza virus, and can provide immunity throughout the season of influenza. The complications of influenza result in more than 200,000 patients in the U.S. to be hospitalized every year.