148 A MANUAL OF BACTERIOLOGY 



to a disease process can be said completely to be demon- 

 strated : 



(1) The organism in question must be present in the 

 tissues, fluids, or organs of the animal affected with, or 

 dead from, the disease. 



(2) The organism must be isolated and cultivated out- 

 side the body on suitable media for successive generations. 



(3) The isolated and cultivated organism, on inoculation 

 into a suitable animal, should reproduce the disease. 



(4) In the inoculated animal the same organism must 

 be found. 



To these may be added : 



(5) Chemical products with a similar physiological 

 action may be obtainable from the artificial cultures of the 

 micro-organism, and from the tissues of man or animals 

 dead of the disease. 



(6) Specific serum and other reactions, agglutinative, 

 bacteriolytic, complement fixative, etc., are generally 

 obtainable, under certain conditions, if the blood of the 

 infected person or animal be allowed to act on the specific 

 organism producing the infection. 



It is true that one or more of these conditions may not 

 be fulfilled in all cases, but on general evidence the disease 

 is classed as infective. 



The modes of infection, or entrance of the infective 

 agent into the body, are varied. The infective agent 

 may enter by (1) the gastro-intestinal tract, e.g. typhoid, 

 cholera, and glanders ; (2) the respiratory tract, e.g. 

 pneumonia and influenza, and occasionally typhoid, 

 plague, etc. ; (3) by inoculation, not necessarily only of 

 the skin, but also of the mucous membranes, e.g. the 

 septic diseases, glanders, tetanus, etc. The extreme infec- 

 tivity of some diseases e.g. variola, scarlatina, influenza, 

 etc. may be due to the fact that infection takes place 

 by the respiratory tract. In certain instances the 



