PARASITES 437 



prevention for one bacterial disease, it should be easy to work 

 out similar methods for all other diseases. This is far from be- 

 ing true. To understand this, you must remember that these 

 different diseases are caused by different species of bacteria 

 and that these different species vary very widely in their char- 

 acteristics and in their relations to the animal body. For 

 example, not all disease-causing bacteria produce a soluble 

 toxine which can be easily separated from the organism and used 

 in the production of antitoxine as in the case of diphtheria. 

 Again, the antitoxines, when produced, are not always so ef- 

 fective in the neutralization of the toxine or so harmless to the 

 body of the patient as is that of diphtheria. Many other 

 differences exist which can not be explained here because of the 

 technicalities and difficulties they involve. It is enough to 

 say here that almost every bacterial disease presents its own 

 peculiar difficulties and that little headway has been made up 

 to the present time in dealing with some of them. 



509. Prevention and Cure. In general, it may be said that 

 the efforts of bacteriologists in seeking to gain control over 

 bacterial diseases are directed along three general lines. 



First, they seek to find some method of preventing the dis- 

 ease or of giving the people or animals artificial immunity 

 from the disease. As a result of efforts along this line 

 we have the various vaccines, such as Pasteur's vaccine 

 against anthrax which consists of attenuated cultures of the 

 bacteria, or the cow-pox virus as a vaccine against smallpox. 

 When vaccines are used, you will note that the body of the 

 patient is stimulated by the presence of the toxine or the at- 

 tenuated bacteria to secrete its own antitoxine or to raise its 

 resistance to the disease in some other way. 



Second, the bacteriologists seek to find some method of pro- 

 ducing an antitoxine and therefore a cure for the disease as 

 in the case of diphtheria and some others that will be men- 

 tioned later. 



Third, bacteriologists seek to learn the methods by which 

 the different diseases are ordinarily spread from patient to 



