organism form elementary body BTO:0000377 BrendaTissueOBO The infectious form of chlamydiaceae. Infection occurs when the small, rigid-walled extracellular form (elementary body) enters the cell and changes into a larger, thin-walled form (initial body) that divides by fission. The daughter cells thus formed reorganize and condense to become elementary bodies that then infect other cells. The organisms are parasites of humans and other vertebrates, capable of producing a variety of diseases. They have also been found in arthropods. The family contains the genus Chlamydia.