Introduction 9 



become incapable of reproducing themselves in sufficient numbers. 

 Xow it is proved that many Insects inoculate pathogenic micro- 

 organisms and thus transmit destructive diseases. A most for- 

 midable epizootic disease, provoked by a flagellated Infusorium, the 

 Trypanosoma brucei, is inoculated into large mammals in South 

 Africa by a fly, the Tsetse fly; in certain districts this disease is so 

 widespread and so destructive that the rearing of domestic animals 

 becomes impossible. 



Parasites strike then with great intensity, bringing about the 

 destruction of numerous human beings, animals and plants. Never- 

 theless, in spite of the disappearance of a large number of species, 

 the world continues well populated. This fact proves that, by the 

 special means at the disposal of the organism, without any aid of the 

 medical art or special human intervention, many living species have 

 held their own throughout the ages. Everybody has seen how dogs 

 lick their wounds, moistening them with a saliva full of micro- 

 organisms. These wounds heal well and quickly without dressings 

 or antiseptics. 



In all these examples the resistance of the organism depends 

 on immunity, a condition very general in nature. This immunity 

 against infective diseases is very complex and its thorough study 

 could only be undertaken after we had acquired an extended knowledge 

 of these diseases, and after adequate methods of research had been 

 devised. 



By immunity against infective diseases we understand the re- [10] 

 sistance of the organism against the micro-organisms which cause 

 these diseases. We have here to do with an organic property of 

 living beings and not with the immunity which belongs to certain 

 countries or localities. For this reason information on the causes 

 of the immunity in Europe and in mountainous regions from yellow 

 fever will not be found in this book, nor why the majority of 

 Europeans do not take recurrent fever. The inhabitants of our 

 continent do not possess organic immunity against either the virus 

 of yellow fever or Obermeyer's spirillum of recurrent fever. Indeed 

 they are very susceptible to these diseases. It is solely the con- 

 ditions of life, in the majority of European countries, that prevent 

 the invasion by the specific germs and the creation of epidemic 

 foci. The same point of view ought also to be applied to animals. 

 Our small laboratory rodents, mice and guinea-pigs, are much more 

 susceptible to anthrax, whether inoculated beneath the skin or 



