Influenza, also known as the flu, generally refers to a group of diseases characterized by sore throat, cough, and fever, which occur after the human upper respiratory tract is infected by the influenza virus. We call it influenza or flu. Since influenza is a viral infection, it has a certain self-limiting nature, meaning that patients might recover completely over time without treatment. The fever caused by influenza usually lasts three to five days for people with normal immune systems. However, for some elderly, infants, and pregnant women, the fever might extend to five days or even ten days, and special attention is needed for these groups.