Theoretical epidemiology refers to the study of the epidemic law and mechanism of infectious diseases, chronic diseases and other group diseases in a specific population based on mathematical models and statistical methods. Theoretical epidemiology is widely used, such as providing scientific basis for disease prevention and control; Forecast the epidemic trend, scale and influence scope of the disease in the future; Evaluate and formulate public health policies; Explore the epidemic mechanism of diseases and analyze the risk factors of diseases. Theoretical epidemiology plays an active role in disease outbreak, post-outbreak control and prevention, and public health policy formulation.