Epidemiologic history is the history of epidemiologic contact, mainly including the history of travel and residence in the infected area, the history of contact with similar patients, and the history of animal contact. 1. History of traveling and living in infected areas: mainly ask patients what places they have been to recently. Different epidemics have different infected areas, according to the disease suffered by the patient should focus on asking whether there is a history of travel and residence in the infected area of the disease. 2. History of contact with similar patients: mainly ask the patient what people have been in contact with recently, whether these people are from the infected area, or with the patient there are similar clinical symptoms. 3. Animal contact history: mainly ask the patient what animals have been in contact with recently, mainly used in the investigation of zoonotic diseases, such as avian influenza, plague and so on. In the event of infectious diseases, according to the epidemiological history of infectious patients and the epidemiological characteristics of the corresponding diseases, the patient can be inferred or diagnosed source of infection, and can cut off the chain of transmission in a timely manner, to control the spread of infectious diseases.