174 PUBLIC HEALTH BACTERIOLOGY 



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They are called specific, because they have a perfectly 

 definite course, characterized by the stages of incubation, 

 invasion, advance and death, or decline and convalescence. 

 Some of them have also a skin eruption or rash. 



The infection is given off again by the breath, exhalations 

 from the skin and wounds, by desquamated portions of 

 the epidermis, by the secretions and excretions (mucus of 

 mouth and throat, saliva, sputa, faeces, urine, seminal 

 fluid, milk). 



Micro-organisms, then, are only relatively pathogenic or 

 non-pathogenic, and in any particular instance of 

 pathogenicity, the amount and kind of attack vary with 

 a large number of factors. This is not a matter for wonder 

 now, with our knowledge of the varied needs and differences 

 in vitality of many micro-organisms, but was a stumbling- 

 block in the early days. Following Muir and Ritchie, we 

 may summarize the matter thus : 



Infection is conditioned by (i) the infecting agent, and 

 (2) the subject. 



1. The infecting agent produces its effect dependent 



on (a) its virulence, (b) its numbers, (c) its path 

 of entrance. 



2. The subject varies in its susceptibility or the 



reverse (resistance), according to (a) its species, 

 (b) race, (c) age, (d) individual peculiarities, 

 (e) vitality, (/) other disease. 



Mode of Action. Multiplication ; invasion of lymphatics ; 

 invasion of blood-stream ; settlement in certain tissues ; 

 chemical products (toxins), locally or diffused. 



Effects of Bacterial Action. 



1. Tissue changes : 



(a) Local tissue reactions or degeneration and 



necrosis, acute or chronic. 



(b) Distant damage to special tissues, reaction of 



blood-forming organs. 



(c). General malnutrition or increased waste, or 

 both. 



2. Metabolic changes : fever, etc. 



