80 VACCINE AND SERUM THERAPY 



manifestations of a general infection and hemorrhagic 

 inflammatory processes of the internal organs. The 

 bipolar bacilli are very widely spread, and may pro- 

 duce disease in any species of animal, and more 

 especially the domesticated ones. Originally sapro- 

 phytes, they occur in nature in the soil, in stagnant 

 water, on various plants, and in all kinds of dead 

 organic matter. Under normal conditions they do 

 not manifest pathogenic characteristics. Under certain 

 conditions, which at present are still unknown, they 

 become very virulent, and then may attack the 

 healthy tissues of the animal body. On the other 

 hand, their pathogenic action is favoured by all those 

 influences which weaken the body resistance. Some 

 of the organisms which have become parasites and 

 pathogenic, after leaving the body again return to 

 their saprophytic harmless existence, but in the 

 majority of cases, once they have gained a parasitic 

 existence, their virulence remains exalted for the 

 particular species of animal they have infected, and 

 by their dissemination are liable to spread the disease 

 among the particular species of animal. This relative 

 virulence may even increase for a time in the course 

 of further generations, or may become constant. 

 More frequently, however, it diminishes again after 

 a certain time. These characteristics of the bipolar 

 bacteria explain the variation which is observed in the 

 appearance and spread of the diseases belonging to 

 this group. Commonly these diseases appear in a 

 locality periodically, without any apparent connection 



