It is an infectious disease caused by an influenza which is called influenza virus. This virus is in the patient's nasal mucus, saliva, or sputum. It can contact by coughing or touching surfaces contaminated by the virus. The incubation period of influenza is 1-4 days. There are three types of influenza viruses affect people, called Type A, Type B, and Type C. Each type can be separated in many sup-species. In each disease is caused by a single sup-species which is already immune to that strain. Moreover, some species may take turns rotating to cause large outbreaks and epidemics; the disease is called each time by the name of the country which is the origins of the outbreak of the flu worldwide.