 So, although the concepts of immunity were developed primarily in the context of disease ecology, attention is shifting to symbiotic ecosystems. The exchanges the immune system mediates highlight context sensitivity of parasitism, commensalism and mutualism, and further demonstrates the bidirectional dialogue between bacteria and the host in the mutual construction of their respective ecological niches. The problem of immune evolution and clonal selection of lymphocytes is considered, in turn, to provide the most general account of natural selection. An important new issue concerns a set of problems associated with the recently discovered adaptive immune system of bacteria, the CRISPR system, whose role in evolution poses novel challenges to basic Darwinian principles (see section 3.2).