Host-microbe relationships in chronic respiratory infection.

Infection of the upper and lower respiratory tracts accounts for 85% of the respiratory disease seen by primary-care physicians. Acute infection is usually attributable to microbial virulence but may occur on a background of immune deficiency. Recurrent acute bronchial and pneumonic infection is associated with considerable immunological abnormality (up to 75%) for which a high index of suspicion must be maintained. Chronic bronchial sepsis, on the other hand, has a paradoxically low prevalence of immune deficiency (less than 10%) and the pathogenesis depends on initially compromised mucociliary clearance (by exogenous agents or underlying genetic disease) allowing certain microbes to be selected for airway colonisation according to their ability to release cilio-inhibitory factors and factors damaging ciliated epithelium. Once microbial colonisation is established, the host responds exuberantly with non-specific and immune inflammatory responses which fail to clear the microbial flora but damage the 'innocent bystander' lung. This further compromises bronchial clearance mechanisms in a 'vicious circle' of events whose end result is progressive lung damage and cardio-respiratory failure.