Antibodies may be produced after the flu is cured, but they are not permanent, and the antibody titer will gradually decrease until it disappears. Influenza is caused by infection with influenza viruses, which are categorized as influenza A, influenza B, and influenza C. Influenza viruses are prone to mutation, and therefore pandemics often occur worldwide. Normally, after the body is infected with influenza virus and cured, it will acquire protective antibodies against the corresponding subtype of influenza virus, and will not become ill again if it is exposed to the virus again within a short period of time. However, due to the high mutability of influenza viruses, re-exposure to other subtypes of influenza viruses may result in infection due to the lack of antibodies in the body. Therefore, it is recommended that people who need to prevent influenza should receive influenza vaccination before the annual influenza epidemic season.