Respiratory pathogen invasion and the host immune response. Dysbiosis is developed by increased colonization of pathogens into the upper respiratory tract (URT), shifting the structure of the URT microbiome and then proliferating and infecting the lungs. In the pre-BRD state, the mucosal barrier’s function responds to dysbiosis of the bacterial community and the reproduction of pathogens across the airway epithelium by releasing chemokines and cytokines in mucus production and activates local immune cells . The onset of BRD occurs when the unbalanced pre-BRD state suffers a decline into the clinical exacerbation state. Then, detectable damaged epithelium accompanied by a functional deficiency of the mucosal barrier is commonly found in the respiratory tracts of BRD calves.