INTRODUCTION. 27 



tributed gratuitously to farmers by the United States gov- 

 ernment. Inoculation of human subjects with the atten- 

 uated living virus of a disease is used only for hydrophobia. 

 This method also was invented by Pasteur. 



The preparations of antitoxins for infectious diseases 

 (see the chapter on Immunity) 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 

 microscopic parasites whose nature makes their study almost 

 inseparable from that of the bacteria. In this class also 

 belong the primitive forms of animal life (Protozoa) which 

 are the causes of amebic dysentery (Losch, 1875) and 

 malaria (Laveran, 1880). The disease of cattle called 

 " Texas 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 an 

 insect, the cattle-tick. Since then it has been shown (by 

 Manson, Ross and others) that malaria is conveyed from a 

 person having the disease to one not affected by means of 

 mosquitoes. It now appears probable that a similar rela- 

 tion exists between 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 have larger 

 possibilities in store. 



It is encouraging to reflect that the progress of bacte- 

 riology has been made by gradual and logical steps. The 

 great discoveries have not been lucky accidents, but have 

 been worked out patiently and with deliberation. 



