ABSTRACT
I. Introduction The human airways below the vocal cords are sterile as a result of innate immune defenses. The development of pulmonary infection occurs with either a defect in host defenses, a particularly virulent pathogen, an overwhelming inoculum, or a combination of these three factors. A feature of many bacterial respiratory tract pathogens is that under usual circumstances, they colonize the upper respiratory tract without causing clinical symptoms. The development of clinical infection represents a perturbation of the symbiosis between colonizing bacteria and the host.
