A longitudinal study will be conducted involving 120 heifer calves from a single dairy herd, observed from birth to weaning. Samples including nasopharyngeal swabs and tracheobronchial lavage (TBL) will be collected during the early weeks of life, as well as from calves showing signs of respiratory disease and matched healthy controls. Blood samples will be taken at various points, including post-recovery. Nanopore microbiota and virome sequencing will be performed on the collected samples, and inflammatory markers will be analyzed.