A susceptible population is a group of people who lack sufficient resistance to a disease-causing agent, and a group composed of susceptible people is called a susceptible population. Usually, in the control of infectious diseases, the first step is to protect the susceptible person, who is usually a person who lacks specific immunity to an infectious disease and is vulnerable to infection. Individuals in the population as a whole, the resistance of susceptible people is ultra-low, extremely susceptible and the proportion of highly susceptible people in the population to a certain level, there are sources of infection and suitable transmission routes, it is easy to occur epidemics of infectious diseases. The epidemic of infectious diseases must have three links, that is, the source of infection, the transmission route and the susceptible population, the three links must exist at the same time, the absence of one cannot constitute an epidemic of infectious diseases, such as a disease, such as tuberculosis susceptible population is the immunocompromised elderly, HIV-infected people, and the application of immunosuppressive drugs, known as the susceptible population of tuberculosis.