Pseudomonas aeruginosa, in spite of being an ubiquitous microorganism, rarely colonizes healthy individuals. Hospitalization, as well as the length of time spent in hospital, significantly increase the rate of colonization, especially in immunocompromized patients. Antibacterial chemotherapy or prophylaxis together with invasive diagnostic procedures favour colonization and infection with P. aeruginosa. Immune deficiencies and adverse interactions of P. aeruginosa with the immune system facilitate infection. Thus, a number of exogenous as well as endogenous factors and their interactions favour infection of patients with P. aeruginosa.