Infection is regarded as an imbalance between microbial pathogenicity factors and the host defense system. The virulence mechanisms include adhesion, chemotaxis, invasion, resistance and production of toxins. In addition, local and/or systemic immune functions in patients undergoing infections are altered. Nonspecific as well as specific cellular and humoral defense mechanisms are affected. The interaction of defined microbial pathogenicity factors with immune effector cells results in the activation of a variety of inflammatory mediators; they are a prerequisite for protective immunity but also induce local or systemic damage in the host when they occur in excessive amounts and when their metabolism is inadequately controlled. The analysis of the pathophysiological events during infection in patients by taking advantage of modern molecular and cell-biological methods may contribute to the development of novel therapeutic strategies.