INTRODUCTION. 1 7 



But not only have inoculations against the microorganisms 

 themselves been perfected in some cases, but also the sub- 

 stances which are called antitoxins, the specific agents against 

 the poisonous products of the bacteria, have been produced. 

 The discovery of these antitoxins for infectious diseases 

 (see Part II., Chapter VII.) we owe to Behring. This portion 

 of our subject belongs entirely to the present day, and is now 

 being studied with great energy. 



Allusion has already been made to moulds and other micro- 

 scopic parasites whose nature makes their study almost in- 

 separable from that of the bacteria. In this class also belong 

 the primitive forms of animal life, the Protozoa, which are the 

 causes of amebic dysentery (Losch, 1875) and malaria (Lav- 

 eran, 1880). The disease of cattle called "Tex.as fever" is also 

 caused by a protozoon. Theobald Smith in the United States 

 discovered that the parasite of Texas fever is conveyed from 

 one animal to another by the cattle-tick. Since then it has 

 been shown (by Manson, Ross and others) that malaria is 

 conveyed by mosquitoes from a person having the disease to 

 one not affected. It has also been shown by Reed and 

 Carroll that a similar relation exists bfetween mosquitoes and 

 yellow fever. The part played by flies and other insects in 

 carrying disease germs is still receiving active attention and the 

 future may show that these play a most important part in 

 diseases other than those already mentioned'. 



In 1903 Novy and McNeil succeeded in making pure cul- 

 tures of pathogenic protozoa grow in tubes, in nearly the same 

 way that cultures of bacteria are propagated (see Appendix). 



It is encouraging to reflect that the progress of bacteriology 

 has been made by gradual and logical ^teps. The great dis- 

 coveries have not been lucky accidents, but have been worked 

 out patiently and with deliberation. 



