Betacoronavirus; Coronavirus Infections/complications; Gastrointestinal Diseases/etiology; Gastrointestinal Tract/virology; Pandemics; Pneumonia, Viral/complications; COVID-19; COVID-19 Testing; Carrier State; China/epidemiology; Clinical Laboratory Techniques; Comorbidity; Coronavirus Infections/diagnosis; Coronavirus Infections/epidemiology; Coronavirus Infections/transmission; Feces/virology; Gastrointestinal Tract/physiopathology; Hepatitis, Viral, Human/epidemiology; Hepatitis, Viral, Human/etiology; Humans; Immunosuppressive Agents/adverse effects; Immunosuppressive Agents/therapeutic use; Inflammatory Bowel Diseases/drug therapy; Inflammatory Bowel Diseases/epidemiology; Neoplasms/epidemiology; Pneumonia, Viral/epidemiology; Pneumonia, Viral/transmission; Practice Guidelines as Topic; SARS-CoV-2; Saliva/virology