Pseudomonas aeruginosa is an opportunistic human pathogen that in most clinical situations is of low virulence. In the healthy host, inhaled bacteria are cleared rapidly by an arsenal of innate defence mechanisms including mucociliary clearance and immunological in nate or acquired responses. In some clinical situations Pseudomonas aeruginosa is an opportunistic human pathogen that in most clinical situations is of low virulence. In the healthy host, inhaled bacteria are cleared rapidly by an arsenal of innate defense mechanisms including mucociliary clearance and immunological in nate or acquired responses.