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Endosymbiosis describes the situation in which one organism lives within cellss of another organism. The intracellular organism is called an endosymbiont (see this article for examples of endosymbionts in nature).

It is also generally believed that certain organelles of eukaryotic cells, especially mitochondria and chloroplasts, originated as bacterial endosymbionts. This theory is known as the endosymbiotic hypothesis.