Elie Metchnikoff's and Paul Ehrlich's impact on infection biology.

The 100th anniversary of the Nobel Prize for Medicine to Paul Ehrlich and Elie Metchnikoff gives us a good opportunity to reflect on their research about infectious diseases. Elie Metchnikoff was not only the first to describe phagocytosis of invading pathogens by specialized blood cells - macrophages and neutrophils - he also was interested in the impact of normal flora on well-being and in pre- and probiotic diet and their influence on the normal flora. Paul Ehrlich not only developed the concept of the side-chain theory of antibody formation but also discovered the first chemotherapeutic agent against microbial pathogens through a combination of chemical modification of a lead substance and experimental animal screening on a broad-scale. Hence, they are not only the founders of immunology but also were the first to envisage infection biology as the result of an interplay between host and pathogen.