1\. To research the current situation of severe infection in children in China, and to investigate the incidence, prognosis and disease burden of severe infection in children in different regions of China. 2. Establish the risk prediction model and diagnosi model of severe infection in children, and verify the accuracy of the model in multi-center; 3. To study the effectiveness and safety of different treatments in real diagnosis and treatment, and to evaluate the efficacy of subgroups under different ages and high risk factors.