Effects of Probiotics on Respiratory Tract Infections in Children: A Randomized, Placebo-Controlled Interventional Study

Inclusion Criteria: * Children aged 3 months to 6 years, regardless of sex. * Meet the clinical diagnostic criteria for upper respiratory tract infection, including but not limited to cough, rhinorrhea, nasal congestion, sore throat, and fever. * Assessed by a clinician as having mild or moderate URTI, not requiring antibiotics or other systemic prescription medications, and suitable for observation and/or symptomatic management. * Willing to discontinue other probiotics or supplements during the study period, except for the study product provided by the investigators. * Agree to complete questionnaires and provide fecal biological samples during the study period. * The child's parent(s) or legal guardian(s) are fully informed about the trial and voluntarily sign the written informed consent form before any study procedures begin. Exclusion Criteria: * Severe URTI as judged by the clinician, or symptoms severe enough to require antibiotics, corticosteroids, nebulized steroids/bronchodilators, or other systemic prescription medications, including suspected or confirmed bacterial infections such as bacterial tonsillitis, bacterial sinusitis, or acute suppurative otitis media. * Acute lower respiratory tract infections, such as acute bronchitis, acute laryngitis/laryngospasm, or pneumonia. * Respiratory tract infections secondary to underlying diseases, including primary immunodeficiency, acquired immunodeficiency syndrome, congenital airway malformation, congenital heart disease, gastroesophageal reflux, or abnormal lung development. * Severe malnutrition. * Severe chronic diseases, including severe asthma or other chronic respiratory diseases, chronic hepatitis or other liver diseases affecting liver function, renal insufficiency or other severe kidney diseases, neurological diseases such as epilepsy or neurodevelopmental disorders, and major genetic diseases such as chromosomal abnormalities or monogenic disorders. * History of epilepsy or febrile convulsions. * Planned vaccination during the trial period. * Severe infection, severe trauma, or history of medium-to-major surgery within the past month, as judged by the investigator. * Participation in other clinical intervention studies involving drugs, dietary supplements, probiotics, or prebiotics within the past 3 months. * Known allergy to any component of the probiotic product. * Any other condition that, in the opinion of the investigator, makes the participant unsuitable for this clinical study.