This project will study severe COVID-19 related factors, host genes, biomarkers, and prognosis of severe encephalitis in children, and conduct: 1. Retrospective clinical analysis of children with COVID-19, to analyze the clinical manifestations of children with new crown infection, and to find the relevant factors of severe disease. 2. Prospective acceptance in the acute phase: mainly to understand the correlation between the interaction of host factors, viruses and microorganisms and the severity. 3. Children's long COVID-19 Tracking Team An integrated outpatient clinic for Long COVID-19 Syndrome in Children will be established to conduct systematic tracking of body, lung function, nerves and psychology. Children with neurological complications or other confirmed diagnoses will undergo physical, neurological, pulmonary function and brain MRI and neurophysiological examinations, sleep examinations, and conduct psychiatric clinical diagnostic interviews to establish a psychiatric diagnosis. Status and family social function, the intelligence development status was examined by intelligence test, and cognitive function was measured by neuropsychological test. Understand the microbial phase, immune function and incidence of allergic diseases after long-term new coronavirus infection. 4. Seroepidemiological Study Group Exploring the seroepidemiology of different groups of health care workers and non-health care workers will allow us to better understand the magnitude of natural infection among children and age groups in the community and the infection profile of health care workers during this period. This trial includes multiple groups of prospective cases to explore the impact of the new coronavirus on children and the mechanism of severe illness, and integrates various specialties and branches of the Children's Hospital to conduct a summary trial, which is expected to understand children's new crown and community epidemiology, children infected with new crown Clinical manifestations of the virus, risk factors and effects on various systems, combined with host microbiome, immune function analysis and whole-exome sequencing, to improve the detection of new coronavirus (including long-term new coronary syndrome and pediatric multisystem inflammatory syndrome). Mechanisms of host factors and interactions with microbes to improve outcomes and severe incidence in children.