18 THE ETIOLOGY OF THE BACTERIAL DISEASES. 



Some species of single-celled animal organisms, known as pro- 

 tozoa, may invade the body and there live and reproduce themselves, 

 modifying, impairing and destroying normal tissue. The disorders 

 resulting from these causes are known as protozoal diseases. 



Other more highly developed animals pass at least a portion of 

 their lives as parasites, and we must recognize certain diseases as due 

 to animal parasites. 



The living cells of the animal body may be altered or destroyed 

 by the action of poisons of mineral, vegetable or animal origin. 

 The poisoning that results in this way may be acute or chronic ; it 

 may manifest itself in one case principally by its action on the nerv- 

 ous system, and in another the symptoms induced may be referred 

 more especially to the digestive organs. Diseases due to the ad- 

 ministration of poisons generated wholly outside the body are grouped 

 together under the name of intoxications. 



A given group of cells in the body may be so altered by mechan- 

 ical violence that the continued performance of healthy function is 

 no longer possible. A depression of the skull, as the result of a fall 

 or blow, may induce epilepsy or insanity. Diseases induced in this 

 manner are said to be traumatic. 



Lastly, without outside interference, any group of cells in the 

 body may, from having an excess of work thrown on it, or from 

 other causes, many of which remain unknown, fail to do its duty, 

 and, as a consequence, disaster may threaten the whole. These 

 diseases may be denominated as autogenous. 



This gives us a simple etiological classification of diseases into : 

 (1) Bacterial, (2) fungous, (3) protozoal, (4) animal parasitic, (5) 

 intoxications, (6) traumatic, (7) autogenous. 



While the above given etiological classification of diseases is ad- 

 missible, it must be understood that in many instances the cause is 

 not single, but multiple, and for this reason sharp lines of classifi- 

 cation cannot be drawn ; for instance, the greatest danger in those 

 traumatic affections in which the traumatism itself does not cause 

 death, lies in infection. The wound has simply provided a suitable 

 point of entrance for the infecting agent ; indeed the break in the con- 

 tinuity of tissue may be so slight that it is of import and danger only 

 on account of the coincident or subsequent infection, as is true in 

 most cases of tetanus and septicemia. Furthermore, an infectious 

 disease, whether it originates in a traumatism or not, is markedly 

 influenced by what we are pleased to call the idiosyncrasy of the 

 patient, and by this we mean the peculiarities of tissue metabolism 

 taking place in the individual at the time. A dozen men may be 

 alike exposed to the same infection, and the infecting agent may find 

 a suitable soil for its growth and development in two of these, while 

 in the other ten this same agent meets with such adverse influences 

 that it dies without producing any appreciable effects ; or all may be 



