The immunocompromised host is an individual whose defense mechanisms against infectious agents are altered in such a significant way that he is abnormally susceptible to infections agents in general and bacterial "opportunists" in particular. The type of infection and etiologic agents vary with the nature and the severity of the immune defect. The goal of prophylactic treatments is to prevent the occurrence or recurrence of infections, and so, to reduce the infectious morbidity and mortality when the nature of severity of immunosuppression make these complications probable. Beside specific measures, which are detailed in this article, the strict application of the so-called universal precautions, particularly handwashing, remain at the basis of the prevention of bacterial infections in the immunocompromised host.