Pathogenesis of bacterial infections of the respiratory tract.

The main function of the lung is gas exchange, which means that the respiratory mucosa is constantly exposed to noxious substances and potentially infective agents inhaled from the environment. Despite this, in health the number of bacteria in the upper respiratory tract (nose and pernasal tissue) is low and the lower (bronchopulmonary) respiratory tract is sterile. However, respiratory tract infections are common in humans, either due to malfunction of a complex array of defence mechanisms or because of the virulence of a particular microorganism. The pathogenesis of these infections is discussed.