The name Chlamydozoa was proposed by Prowazek in 1907 for a number of minute, problematic organisms (fig. 119) believed to be the causal agents of certain diseases in man and animals, such as vaccinia and variola, trachoma, inclusion blenorrhœa in infants, molluscum contagiosum, and bird epithelioma contagiosum. Other diseases possibly due to Chlamydozoa[251] are hydrophobia, measles, scarlet fever, foot-and-mouth disease, the “Gelbsucht” disease of silkworms, and perhaps even typhus (Prowazek, 1913). The subject is difficult and controversial and can only be briefly discussed here. It is known that the viruses in all these diseases can pass through ordinary bacterial filters, that is, they belong to the group of “filterable viruses.” At such periods the organisms are extracellular or free. It is also known that in many of these cases the virus produces definite and characteristic reaction-products or cell-inclusions in the infected cells, during the intracellular phase of the life-history of the organism. As the organisms to be considered are problematic, it will be convenient to summarize their history:-- [251] For a detailed account of the Chlamydozoa see Prowazek’s _Handbuch der Pathogenen Protozoen_, Bd. i (1911–12). Leipzig, J. A. Barth. (1) Cell-inclusions, usually named after their discoverers, have been found in certain diseases, thus: In vaccinia Guarnieri’s bodies, in scarlet fever Mallory’s bodies, in hydrophobia Negri’s bodies, in trachoma Prowazek’s bodies occur. (2) At first these characteristic cell-inclusions were considered to be actual parasitic organisms causing the diseases in question. The bodies received zoological names and attempts were made to work out their supposed development cycles. The supposed parasites of vaccinia and variola were referred to a so-called genus _Cytoryctes_, those of hydrophobia to _Neuroryctes_, of scarlet fever to _Cyclasterium_, while those of molluscum contagiosum were referred to the Coccidia. Calkins in 1904 studied in detail the cell-inclusions of vaccinia and small-pox, calling them _Cytoryctes variolæ_, Guarnieri. Calkins considered that in the stratified cells of the epidermis they passed through two cycles, the one cytoplasmic, the other intranuclear. The first is the vaccinia cycle, the second the pathogenic (intranuclear) variola cycle. It is hardly necessary to follow all Calkins’ stages here. Negri (1909) described a cycle for _Neuroryctes hydrophobiæ_. Calkins refers both _Cytoryctes variolæ_ and _Neuroryctes hydrophobiæ_ to the Rhizopoda. Siegel (1905) described quite different organisms under the name _Cytorhyctes_. He listed several species: _C. vacciniæ_; of vaccinia and small-pox, _C. scarlatinæ_ of scarlet fever, _C. luis_ of syphilis (this is probably the granule stage of _Treponema pallidum_), and _C. aphtharum_ of foot-and-mouth disease.