CHAPTER X 

 THE FILTERABLE VIRUSES 



This general term means that the virus of a disease can pass 

 through a porcelain filter and usually that it cannot be seen by the 

 microscope. It, however, does not mean that it is invisible at all 

 stages since in one case at least we have been able by means of the 

 ultramicroscope to see what is almost certainly the particular 

 causal agent. Again it is said that spirochaetes when young will 

 traverse porcelain niters. The term will cover in this chapter 

 those diseases of importance to man whose causal agents cannot 

 be morphologically described, but whose characters are more or less 

 well known. The list of diseases caused by submicroscopic agents 

 is as follows: African horse sickness, swamp fever of horses, catar- 

 rhal fever of sheep, yellow fever, Dengue, three-day fever, typhus 

 fever, poliomyelitis, rabies, variola, with its congeners vaccinia 

 and animal pox, hog cholera, foot and mouth disease, fowl plague, 

 fowl diphtheria, transplantable sarcoma and leukemia of fowls, 

 cattle plague, trachoma, pleuropneumonia of cattle, molluscum 

 contagiosum, measles, scarlet fever, guinea-pigs epizootic and 

 some diseases of plants. As said above, only the diseases trans- 

 missible to human beings are reviewed. 



Some of the above diseases, notably rabies, scarlatina and tra- 

 choma, show in the leucocytes and epidermal cells certain struc- 

 tures or inclusion bodies to which the name Chlamydozoa was given 

 by Prowaczek, a term implying that a parasitic body is growing in 

 a mantle. They start as tiny specks in the cytoplasm shortly 

 found to be surrounded by a clear, sharply outlined halo. They 

 seem to grow at the expense of the host cell. Their exact char- 

 acter is not understood; they are probably evidences of cellular 

 degeneration under the influence of some noxa. 



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