Pathogenic Bacteria. 6^ 



obtained by preparing a fluid or solid substance in which are 

 present the same constituents as exist in the fluids in which the 

 microbe naturally dwells. We have, accordingl)', various extracts 

 of meat in common use as the so-called media. The extract is. 

 in the first place, in the form of a clear fluid broth, and this may 

 be solidified without losing its clearness by gelatine and other 

 substances. 



If we take such a substance and inject it under the skin of an 

 animal in a moderate dose, it has no harmful effect. If, how- 

 ever, we allow a pathogenic or disease-causing microbe to 

 grow in it for some time before we make the inoculation, we 

 find that the harmless broth has become more or less poisonous. 

 The poisonous effect varies with the form of the microbe, and 

 this difference corresponds with the difference in the diseases 

 with which the microbe is associated. We can therefore pro- 

 duce disease in two ways. Either we can inject the microbe 

 which multiplies in the tissues of the bodv, and so causes the 

 disease, or we can obtain the poison outside the body by grow- 

 ing the microbe and cause the disease, or a condition closely 

 akin to it, by injecting the poison. The microbe flourishing in 

 the tissues of the body produces disease, because it manufactures 

 poisons as it grows, and from the action of these poisons the 

 effects arise. It is of the utmost importance, therefore, for the 

 understanding of the nature of disease to understand the nature 

 of the poisons which are produced in this way, and to ascertain 

 their eflfects on the animal body. The study of these poisons 

 or toxines, as they are often called, has given great definite- 

 ness to the study of infectious disease. 



One aspect of disease in particular, to the explanation of 

 which this study has made important contributions, is that form 

 of resistance to attack which is known as immunity. A man is 

 said to be immune to a disease when he, after exposure to infec- 

 tion, fails to contract the disease. When this power of resistance 

 is an original attribute of the man it is called natural immunity. 

 If, however, by some artificial process, this power has been con- 

 tributed to the man, he is then said to have acquired immunity. 



