CHAPTER VII 



THE PATHOGENIC PROTOZOA : FILTER-PASSING VIRUSES 



Protozoal diseases — Laveran's discovery — Importance of the morphology 

 and the life-cycle — Intracellular protozoa — Heredity in bacterial and 

 in protozoal diseases — -Diseases due to the so-called invisible microbes. 

 — The ultramicroscope — Filtration — Various types of virus capable 

 of passing filters — Microbes of extreme minuteness described in the 

 pustular diseases of the epithelium — Lesions of the infected cells. 



The Pathogenic Protozoa 



The name of Pasteur must be inscribed at the head of this 

 , chapter. It was by his study of pibrine, a protozoal disease 

 of silk-worms, that our ideas on the microbial diseases were so 

 much advanced. 



The studies on anthrax, the labours of Koch and the great 

 discovery of the attenuation of viruses led the new science in 

 the direction rather of bacteriology; the protozoa had even 

 been somewhat forgotten, when in 1880 Laveran discovered 

 among them the cause of malaria. Since that date their 

 importance in pathology has never ceased to grow. 



The methods of research cannot be quite the same as in 

 bacteriology ; they have not the same simplicity as the bacteria. 

 In the case of the tubercle bacillus, the cholera vibrio, and the 

 streptococcus, we practically know only one single constant 

 fixed form for each, and there is no sign of a life-cycle. The 

 majority of the pathogenic protozoa on the contrary go through 

 a cycle in their existejice whose successive forms may be very 

 •diverse and this cycle may take place, not in a single host, but 

 often in two different ones. The discoveries of Ross on the 



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