Endosymbiosis differs from other types of symbiosis because of the intimate and extensive mixing of genomes, genes, and gene products as well as other biochemicals. In the environment, as well as inside and on the surfaces of large and small organisms, like ourselves, microbes constantly interact. The interactions take many forms, including establishment of commensal or parasitic relationships. Because the relationships require close proximity of the cells, each may develop into endosymbiotic relationships. A cell can be phagocytized by another cell of a different type and be dismantled by biochemical processes, thereby becoming food for the phagocytosing cell. This occurs every second of everyday worldwide. In some cases, parts of the genome of the phagocytized cell may be incorporated into the genome of the phagocytizing cell. This generally amounts to incorporation of parts of a gene or a few genes at a time. Occasionally, very large genomic sections may be transferred into the genome of the cell. However, throughout the evolution of life on Earth, there are other interactions that generally begin by one cell being enveloped by another, but the enveloping cell does not degrade and consume the enveloped cell. Occasionally, the host cell maintains the internalized cell and is able to use it. When both cells benet by increased tness of one or both, this becomes an endosymbiotic relationship (Figures 17.1 and 17.2). Many pathogens invade host cells, but do not kill the cells that they invade. In fact, they may survive longer if they allow the invaded cells to survive. In this way, they may cause a reduction of function in the host cell, but the cell survives and continuously provides nutrients to the invading cell. After these relationships continue over evolutionary time scales, the invaders may become obligate parasites, becoming absolutely dependent on the host cells. Again, in some cases, both cells benet either in a mutualistic or commensal association. During this process, the parasites actually begin to lose parts of their genomes (Table 17.1), which is part of the reason for their absolute dependence on the host.