Clinical Research Directory

Inclusion Criteria: * Children from birth through 7 years of age (younger than 8 years at enrollment). * The participant's parent or legal guardian is able to understand the study, voluntarily agrees to participation, and provides written informed consent. * The participant resides in the study area and is expected to be available for long-term follow-up. * For the General Community Cohort: no known serious underlying disease before enrollment. * For the Infection Observation Cohort: either a history of hospitalization for an infectious disease before 3 years of age, with documented information on the pathogen, infection site, or clinical diagnosis; or at least three healthcare visits for respiratory infections during the 12 months before enrollment. * For the Childhood Asthma Cohort: physician-confirmed asthma or highly suspected asthma according to the protocol-defined age-specific criteria. Children younger than 1 year will not be included in this cohort. Exclusion Criteria: * An acute illness within 14 days before enrollment, including respiratory, gastrointestinal, dermatologic, or urinary symptoms or conditions specified in the protocol. * Known primary immunodeficiency, malignant tumor, serious congenital disorder, or another condition that may substantially alter immune status. * Long-term use of immunosuppressive or other immunomodulatory medications, defined as continuous or intermittent use for a cumulative duration of at least 3 months. * A diagnosed coagulation abnormality or bleeding disorder, including coagulation factor deficiency, other coagulation disorders, or platelet abnormalities. * Any other condition that, in the investigator's judgment, makes the child unsuitable for participation.