INTRODUCTION I3 



sites are now known, many of them associated with important 

 diseases. The strict proof of causal relationship to ithe disease 

 has presented greater difficulties here, especially the step of art fi- 

 cial culture. However, the causal relationship of bacteria hav- 

 ing been demonstrated, the probable causal relationship of 

 the protozoa has found more ready acceptance. Cultures of 

 ameba have been obtained by many workers but the successful 

 cultivation of a pathogenic ameba is still questionable. Pure 

 cultures of trypanosomes were obtained by Novy and his pupils 

 (1903-04) and the infections again produced by inoculation with 

 these cultures. 



The transmission of protozoal diseases by insects, first demon- 

 strated by Salmon and Smith in Texas fever, has developed into 

 a subject of prime importance. Malaria and the insect. Anophe- 

 les, sleeping sickness and tsetse fly, Glossina, are ifhportant ex- 

 amples of this relationship. 



Obermeier (1873) described a motile spiral organism in the 

 blood of relapsing fever, the first known parasitic member of a 

 group of very great importance. Very many pathogenic spiral 

 organisms of this general type are now known. Their systematic 

 relationships have not been fully worked out and further knowl- 

 edge is necessary before they can be finally classed with either 

 the bacteria or the protozoa. Artificial culture of these organisms 

 has revealed a close relationship, to anaerobic bacteria, as far as 

 nutritive requirements are concerned, the most successful culture 

 methods being those devised by Noguchi (19 12). Many of these 

 parasites are transmitted by insects and they pass through a 

 somewhat obscure development in the insect carriers, the forms 

 developed being extremely minute (Nuttall, 191 2). These facts 

 suggest a possible relationship of this group of organisms to the 

 filterable viruses. 



Nocard (1899) discovered that the virus of pleuro-pneumonia 

 of cattle would pass through filters impervious to bacteria. The 

 number of recognized filterable viruses has grown appreciably 

 since then and among them are the causes of several very im- 



