MODES OF BACTERIAL ACTION. 161 



sibility of their multiplying and producing disease greatly 

 increased. In this way the favouring part played by fatigue, 

 cold, etc., in the production of diseases of which the direct cause 

 is a bacterium, may be understood. It is important to keep in 

 view in this connection that many of the inflammation-producing 

 and pyogenic organisms are normally present on the skin and 

 various mucous surfaces. The action of a certain organism 

 may devitalise the tissues to such an extent as to pave the way 

 for the entrance of other bacteria ; we may mention the liability 

 of the occurrence of pneumonia, erysipelas, and various suppura- 

 tive conditions in the course of or following infective fevers. 

 In some cases the specific organism may produce lesions through 

 which the other organisms gain entrance, e.g. in typhoid, diph- 

 theria, etc. It is not uncommon to find in the bodies of those 

 who have died from chronic wasting disease, collections of 

 micrococci or bacilli in the capillaries of various organs, which 

 have entered in the later hours of life; that is to say, the 

 bacterium-free condition of the blood has been lost in the period 

 of prostration preceding death. 



The methods by which the natural resistance may be specifi- 

 cally increased belong to the subject of immunity, and are 

 described in the chapter on that subject. 



Modes of Bacterial Action. In the production of disease 

 by micro-organisms there are two main factors involved, namely, 

 (a) the multiplication of the living organisms after they have 

 entered the body, and (b), the production by them of poisons 

 which may act both upon the tissues around and upon the body 

 generally. The former corresponds to infection, the latter is of 

 the nature of intoxication or poisoning. In different diseases 

 one of these is usually the more prominent feature, but both are 

 always more or less concerned. 



I . Infection and Distribution of the Bacteria in the Body. 

 After pathogenic bacteria have invaded the tissues, or in other 

 words after infection by bacteria has taken place, their further 

 behaviour varies greatly in different cases. In certain cases 

 they may reach and multiply in the blood stream, producing a 

 fatal septicaemia. In the lower animals this multiplication of the 

 organisms in the blood throughout the body may be very exten- 

 sive (for example, the septicaemia produced by the pneumococcus- 

 in rabbits) ; but in septicaemia in man, it very seldom, if ever, 



