126 RELATIONS OF BACTERIA TO DISEASE. 



place. The action of one species of bacterium is also often 

 aided by the simultaneous presence of other species. In 

 this case the latter may act simply as additional irritants 

 which lessen the vitality of the tissues, but in some cases 

 their presence also favours the development of a higher 

 degree of virulence of the former. 



These facts established by experiment (and many others 

 might be given), illustrate the important part which local or 

 general conditions of diminished vitality may play in the 

 production of disease in the human subject. This has long 

 been known by clinical observation. In normal conditions 

 the blood and tissues of the body, with the exception 

 of the skin and certain of the mucous surfaces, are bacterium- 

 free, and if a few organisms gain entrance, they are 

 destroyed. But if the vitality becomes lowered their 

 entrance becomes easier and the possibility 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. 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 suppurative 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, diphtheria, etc. 

 It is not uncommon to find in the bodies of those who 

 have died from chronic wasting disease, collections of 

 micro-cocci or bacilli in the capillaries of various organs, 

 which have entered in the latter 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 



