186 VETERINARY BACTERIOLOGY 



constantly, or normally present in the body or on it is termed endog- 

 enous. Traumatic infection frequently occurs through a break 

 in the continuity of the skin or mucous surfaces. Wounds caused 

 by weapons, instruments, and similar objects, the bites of animals 

 and sucking insects such as the mosquito, flea, tick, or bed-bug, 

 may introduce a pathogenic organism. Ordinarily, the skin is an 

 efficient barrier against infection. Microorganisms may occa- 

 sionally enter through the glands or hair follicles. Some bacteria 

 apparently may injure the unbroken mucous surface, as, for ex- 

 ample, the diphtheria bacillus. Certain disease organisms enter 

 through the digestive tract. Bacteria have been shown to pass 

 unharmed through the intestinal walls and to enter the lymph- 

 vessels and the thoracic duct. To what extent this is the common 

 source of infection in certain diseases, such as tuberculosis, is at 

 present a matter of dispute. 



The lungs constitute the infection atrium in pneumonia, 

 probably in many cases of tuberculosis and aspergillosis, and pos- 

 sibly in certain diseases whose cause has not been determined, 

 such as small-pox. 



The genital organs are the common infection atria in the 

 so-called venereal diseases, such as syphilis, chancroid, and gonor- 

 rhea in man, dourine in the horse, and contagious abortion in 

 cattle. Disease organisms rarely pass from the blood of the 

 mother through the placenta to the blood of the fetus. There 

 are comparatively few diseases, therefore, that are inheritable. 

 Syphilis and small-pox are exceptions. 



Types of Disease Produced by Microorganisms. The char- 

 acteristics of the disease produced by various microorganisms, and 

 the pathological changes they bring about, vary as much as do the 

 morphological and cultural characters of the organisms themselves. 

 It is possible, however, to group related types together and find 

 a general basis for classification. 



Specific and Non-specific Infections. A specific infectious 

 disease is one caused in every case by a single species of organism, 

 and characterized by definite clinical characters or symptom- 

 complex and pathological lesions. A non-specific infection is one 

 which may be caused by one of several organisms, and does not 

 possess the definite characters which enable the determination of 



