Human Milk Oligosaccharides, Gut Bifidobacterium, Vitamin D, and Infant Immunity

The goal of this observational study is to learn how certain natural sugar components in breast milk may be linked with vitamin D level, healthy gut bacteria, and immune markers in healthy breastfed infants. The study will include exclusively breastfeeding mothers and their healthy infants aged 1 to 2 months. The main questions it aims to answer are: * Is this natural sugar in breast milk linked with the baby's vitamin D level? * Is this natural sugar in breast milk linked with healthy gut bacteria? * Is this baby's vitamin D level linked with healthy gut bacteria? * Is this natural sugar linked with immune factors in breast milk? Participants will answer health questions from questionnaire, undergo a brief physical examination, allow breast milk and a small baby blood sample to be collected, and provide an baby stool sample. Researchers will test these samples in the laboratory and analyze the results to answer the research questions.