The harmonious interaction of the respiratory ecosystem in healthy cattle. The complex respiratory ecosystem in healthy cattle is harmonious and contains niche specific environmental properties, microbiota immigrations and host-microbiota interactions, which could resist pathogen colonization to some extent. The gradients of physiological features are along the respiratory tract and move from the nasal cavity to the nasopharynx to the trachea and terminate in the lungs. Lower temperature is reported in the nostrils while the lungs reach body temperature. The partial pressures of airway oxygen (pO2) and carbon dioxide (pCO2) have opposing gradients that are regulated by ventilation and gas exchange at the epithelial surface of the airway . Concerning respiration and microaspiration, micro-particles from the external environment enter the upper respiratory tract (URT) and move to the lungs. Equilibrium is achieved when the host and the respiratory microbiota maintain harmonious interaction [15, 37, 108, 109].