This chapter discusses the genetic diversity within the soil environment and some of the mechanisms that generate genetic diversity within soil-dwelling bacteria. It highlights how the physical and chemical (abiotic) properties of the soil and the living organisms (biotic) within the soil might act as selective forces on existing genetic variations, and how they contribute to the evolution of bacterial pathogens in the soil. It talks about the broadly conserved community behaviors intrinsic to the survival of soil bacteria and their relevance to pathogenesis. Many bacterial pathogens of vertebrates are either members or evolutionarily close relatives of the natural soil microflora. Some of these pathogens are able to survive outside their vertebrate hosts, either as soil saprophytes or within invertebrate hosts. In order to survive within the soil, Pseudomonas aeruginosa have had both to adapt to the physicochemical properties of the soil and to escape predation by bacteria-feeding protozoans and nematodes in the soil. Like protozoa, many insects and free-living soil nematodes use bacteria as food. The innate immune response is an immediate first, and for invertebrates the only, line of cellular-based defense against virulent pathogens. The chapter presents examples that illustrate how some highly conserved bacterial community behaviors have allowed some soil microbes to successfully make the transition to human pathogens. Most prokaryotes found throughout the Earth’s soils live not as single planktonic cells, but in populations of biofilms. Most bacteria species exist in the soil as biofilms, an environment that brings many different strains of bacteria into close contact.