Immune and airway defence clearance exists in normal humans and the prevalence is generally low.
Daily oral uncleanliness, paranasal sinusitis, periodontal disease and gingivitis can promote the growth of anaerobic bacteria. At this time, inhaled pathogenic bacteria can cause disease if they are in a state of impaired consciousness such as alcohol abuse, abuse of prohibited drugs, sedation overdose, seizures, general anaesthesia, or if airway defence clearance and systemic immunity are reduced due to triggers such as cold or extreme fatigue.