INTRODUCTION 13 



diseases was suggested. An enormous number of protozoal para- 

 sites are now known, many of them associated with important 

 diseases. The strict proof of causal relationship to the disease 

 has presented greater difficulties here, especially the step of artifi- 

 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 important 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 syste- 

 matic relationships have not been fully worked out and further 

 knowledge is necessary before they can be finally classed with 

 either the bacteria or the protozoa. The discovery of practical 

 methods of artificial culture for these .organisms has been very 

 recent and the most successful methods seem to have been de- 

 vised by Noguchi (1910-12).' Many of these parasites are trans- 

 mitted by insects and they pass through a somewhat obscure de- 

 velopment in the insect carriers, the forms developed being ex- 

 tremely minute (Nuttall, 1912). 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- 



