The epidemic of an infectious disease must have three basic links are the source of infection, the transmission route and the susceptibility of the population. The three links must exist simultaneously in order to constitute an epidemic of infectious disease, the lack of any one of these links, new infections will not occur, it is impossible to form an epidemic. What is a susceptible population A susceptible population is the level of susceptibility or immunity of a population to an infectious disease agent. An increase in the new population, the concentration of susceptible people or entry into infected areas, and the enlistment of new recruits in the army are likely to cause an epidemic of infectious diseases. Immunity acquired after illness, occult infection of the population, artificial immunity, all make the population less susceptible, less likely to infectious disease epidemics or end their epidemics. The epidemiological process of infectious diseases The process of occurrence, spread and termination of infectious diseases in the population is called the epidemiological process of infectious diseases. The basic characteristics of infectious diseases 1, there are pathogens each infectious disease has its specific pathogens, including viruses, rickettsia, bacteria, fungi, spirochetes, protozoa, etc.. 2, there are infectious pathogens from the host out of the body, through a certain way, to reach new susceptible people, showing a certain degree of infectiousness, the intensity of infection with the type of pathogens, the number, virulence, susceptible people’s immune status and other related. 3, there are epidemic, endemic, seasonal (1) epidemic according to the intensity and breadth of the epidemiological process of infectious diseases are divided into. Sporadic: refers to the scattered occurrence of infectious diseases in the population; Epidemic: refers to a region or a unit, in a certain period of time, the incidence of a certain infectious disease, more than the level of incidence of the same period in previous years; Pandemic: refers to the rapid spread and spread of a certain infectious disease in a short period of time, more than the general intensity of the epidemic; Outbreak: refers to a local area or unit, in a short period of time, the sudden appearance of numerous of the same disease patients. (2) Endemic refers to certain infectious or parasitic diseases, which have an intermediate host, and are often limited to a certain geographical area due to geographical conditions and changes in temperature conditions. Such as insect-borne infectious diseases, natural epidemic diseases. (3) Seasonality refers to the incidence of infectious diseases, there is a seasonal increase in the year. This is related to changes in temperature and humidity. 4, there is immunity infectious diseases after recovery, the body does not feel the same infectious disease pathogens, called immunity. Different infectious diseases, the state of immunity after the disease varies, some infectious diseases can be immune for life after one illness, some can also be infected. It can be divided into the following kinds of infection phenomenon. (1) reinfection with the same infectious disease after complete recovery, after a certain period of time, by the same pathogen infection. (2) repeated infection of a disease in the onset of the disease, by the same pathogen re-infested and infected. Schistosomiasis, filariasis, and malaria are the most common. (3) Recurrence of the disease, which has turned into a recovery period or is close to recovery, and the pathogen reappears and multiplies, and the original symptoms reappear. Typhoid fever is the most common. (4) Relapse clinical symptoms have been relieved, but the body temperature has not yet normalized and has risen again, and the symptoms are slightly aggravated. This is seen in typhoid fever.