It is becoming apparent that several intracellular bacterial pathogens of humans can also survive within protozoa. This interaction with protozoa may protect these pathogens from harsh conditions in the extracellular environment and enhance their infectivity in mammals. This relationship has been clearly established in the case of the interaction between Legionella pneumophila and its protozoan hosts. In addition, the adaptation of bacterial pathogens to the intracellular life within the primitive eukaryotic protozoa may have provided them with the means to infect the more evolved mammalian cells. This is evident from the existence of several similarities, at both the phenotypic and the molecular levels, between the infection of mammalian and protozoan cells by L. pneumophila. Thus, protozoa appear to play a central role in the transition of bacteria from the environment to mammals. In essence, protozoa may be viewed as a 'biological gym', within which intracellular bacterial pathogens train for their encounters with the more evolved mammalian cells. Thus, intracellular bacterial pathogens have benefited from the structural and biochemical conservation of cellular processes in eukaryotes. The interaction of intracellular bacterial pathogens and protozoa highlights this conservation and may constitute a simplified model for the study of these pathogens and the evolution of cellular processes in eukaryotes. Furthermore, in addition to being environmental reservoirs for known intracellular pathogens of humans and animals, protozoa may be sources of emerging pathogenic bacteria. It is thus critical to re-examine the relationship between bacteria and protozoa to further our understanding of current human bacterial pathogenesis and, possibly, to predict the appearance of emerging pathogens.