Clostridioides difficile infection (CDI) poses an increasing threat to infant and young child health, with detection rates rising annually. This retrospective study aims to explore the epidemiological characteristics, clinical manifestations, and potential biomarkers of CDI in children aged 0-2 years by examining three cohorts: (1) infants diagnosed with CDI, (2) asymptomatic carriers of C. difficile, and (3) healthy controls. Fecal samples from each group will undergo metagenomic sequencing and metabolomic profiling, coupled with questionnaire-based surveys for risk factor assessment. The findings are anticipated to identify key high-risk factors, elucidate the pathogenic mechanisms underlying infant CDI, and support the development of early diagnostic tools and preventive strategies.

Inclusion Criteria:

1. Age Range: Infants aged 0 to 2 years (inclusive) at the time of sample collection or medical record documentation.

2. Data Availability: Complete medical records or available stool samples within the study's retrospective time frame.

3. Group-Specific Criteria:

   CDI Patients: Documented diarrhea or related gastrointestinal symptoms, with laboratory-confirmed C. difficile by PCR or culture.

   Asymptomatic Carriers: Positive C. difficile test (PCR or culture) in the absence of diarrhea or other clinical CDI symptoms.

   Healthy Controls: Negative C. difficile test and no gastrointestinal symptoms indicative of CDI.

4. Consent/Authorization:Retrospective data (e.g., existing medical records or stored biosamples) may be included under a waiver of consent if approved by the institutional review board (IRB). However, any new information obtained directly from participants or their guardians (e.g., via questionnaires) requires explicit informed consent.

Exclusion Criteria:

1. Incomplete Data: Infants whose medical records lack sufficient information to confirm their CDI status or those without adequate stool sample results.
2. Ambiguous Diagnosis: Patients presenting with other infectious diseases or conditions that could not rule out alternative diagnoses for diarrhea (e.g., confirmed concurrent viral or parasitic infections) without conclusive C. difficile testing.
3. Severe Comorbidities: Infants with life-threatening congenital conditions (e.g., severe immunodeficiency syndromes) if these conditions significantly alter the gut microbiota or confound CDI diagnosis.

This retrospective observational study will include infants aged 0 to 2 years who underwent clinical evaluation and stool testing for Clostridioides difficile at [Hospital(s)/Institution(s)/Region]. The study population is divided into three groups:

1. CDI Patients: Infants with laboratory-confirmed C. difficile (PCR or culture) and clinical symptoms consistent with infection (e.g., diarrhea, abdominal pain).
2. Asymptomatic Carriers: Infants who tested positive for C. difficile but did not exhibit any related symptoms.
3. Healthy Controls: Infants who tested negative for C. difficile and had no history of gastrointestinal symptoms during the same period.

All participants included in this analysis will have relevant medical records and stool sample data available within the study's retrospective timeframe. Additional questionnaire data may be collected from parents or guardians to assess potential risk factors and environmental exposures