144 RELATIONS OF BACTERIA TO DISEASE. 



cocci 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 

 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 in- 

 volved, 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 extensive (for example, the septi- 

 caemia produced by the pneumococcus in rabbits) ; but in 

 septicaemia in man, it very seldom, if ever, occurs to so 

 great a degree, the organisms rarely remain in large numbers 

 in the circulating blood, and their detection in it during 

 life by microscopic examination, and even by culture 

 methods, is rare. In such cases, however, the organisms 

 may be found post mortem lying in large numbers within 

 the capillaries of various organs, e.g., in cases of septicaemia 

 produced by streptococci. (Relapsing fever forms an 

 exception, as in it numerous organisms may be seen in a 

 drop of blood.) In the human subject more frequently 

 one of two things happens. In the first place, the organ- 

 isms may remain local, producing little reaction around 



