98 INFECTION 



the bacilli are inhaled, rubbed into the skin, or reach the lymphatics 

 through superficial abrasions; similarly, local staphylococcus and 

 streptococcus infection may become general, regardless of the route of 

 invasion or the location of the local lesion. 



The avenue of invasion is also of importance in determining the form, 

 nature, and virulence of an infection. Thus virulent pneumococci 

 lodging in the pharynx may produce a pseudomembranous angina; in 

 the eye, a severe conjunctivitis; and in the lungs, a pneumonia. When 

 tubercle bacilli gain admission through the skin, they may produce lupus, 

 or a low-grade inflammatory disease rarely terminating fatally. When 

 inhaled, they may produce tuberculosis of the lungs; in the throat they 

 may reach the tonsils and later the local lymphatic glands, etc. When 

 swallowed, they may produce ulceration of the intestines, or pass through 

 the intestinal walls and involve the mesenteric glands, and later the 

 lungs or other organs. 



Just as general susceptibility of the host renders infection more 

 likely to occur, so local susceptibility may be induced by injury and 

 fundamental disorders. These changes may not only furnish pabulum 

 for the invading bacteria, but more especially reduce the local resistance 

 of the body defenses. 



Even more important, however, is the predisposition of some patho- 

 genic microorganisms to attack certain tissues or organs, and the fact 

 that these tissues are particularly weak in defensive power, so that the 

 bacteria naturally lodge where conditions are most favorable for their 

 growth. 



While the primary focus of infection is determined largely by the 

 route of invasion, the selective affinity of microorganisms or their toxins 

 for certain tissues and the inherent tissue susceptibility to the toxins 

 are best in evidence in the location of secondary foci or localization of 

 the infection in general bacteremias. Thus the seat of the principal 

 local lesions in pneumonia is the lungs, and in typhoid fever the lym- 

 phoid tissues, especially that of the spleen, and Peyer's patches in the 

 intestine. It is true that mechanical factors may aid in this selection, 

 as, e. g., the occlusion by emboli of microorganisms caught in the capil- 

 laries of organs; but, in general, we must conclude that either (1) 

 Microorganisms tend to be destroyed in every tissue or organ except 

 those that are poor in defensive forces and are susceptible, or (2) that 

 microorganisms or their products circulate passively through a tissue 

 and do not lodge because they possess no affinity for these cells. In 

 many infections both processes are probably operative, and at least we 



