70 Infection 



abolic products into the juices, and occasion varying morbid 

 conditions. 



The parasitic bacteria live in habitual association with higher 

 organisms. Sometimes, and indeed most commonly, it is a harmless 

 association, like that of certain cocci upon the skin, but occasionally 

 it results in the destruction of the tissues and the death of the host, as 

 in tuberculosis, leprosy, etc. 



The group of pathogenic organisms has no well-defined Umits, for 

 it is frequently observed that micro-organisms weU known under 

 other conditions, and not known to have been engaged in pathogenic 

 processes, turn up unexpectedly as the cause of some morbid condi- 

 tion. Indeed, although we are acquainted with a large number of 

 organisms that have never been observed in connection with disease, 

 we are scarcely justified in concluding that they are incapable of 

 producing injury should proper conditions arise. 



SOURCES OF INFECTION 



The sources of infection may be exogenous or endogenous; that is, 

 they may arise through the admission to the tissues of micro-organ- 

 isms from sources entirely apart from the individual infected, or 

 through the admission of some of those parasitic and usually harmless 

 organisms constantly associated with him. 



Exogenous infections arise through accidental contact with 

 infective agents belonging to the external world. 



A polluted atmosphere may carry into the respiratory passages 

 micro-organisms capable of colonizing there. From the respiratory 

 passages, minute drops of secretion may be coughed or sneezed into 

 the atmosphere to be inhaled by neighboring persons and infect 

 them. Such "drop infection" has been studied in reference to 

 tuberculosis and diphtheria, and doubtless explains the transmission 

 of whooping-cough, pneumonia, and other respiratory disturbances. 

 Polluted water or food may carry into the intestine micro-organisms 

 whose temporary residence may entirely change the functional and 

 structural integrity of the parts, as in typhoid fever, cholera and 

 dysentery. 



Wounds inflicted by the teeth of animals, by weapons, by imple- 

 ments, or by objects of various kinds, carry into the tissues micro- 

 organisms whose operations, local or general, may variously affect 

 the organism to its detriment. Examples are to be found in rabies, 

 ■tetanus, anthrax, malignant and gaseous oedema, suppuration, etc. 



Fomites, or objects made infective through contact with individuals 

 suffering from smallpox, scarlatina, and other contagious or actively 

 infectious diseases, become the means through which the specific 

 micro-organisms may be conveyed to the well with resulting 

 infection. 



Contact with unclean objects of various kinds — spooiis, knives, 

 cups, blow-pipes, catheters, syringes, dental instruments, etc.— 



