98 INFECTION 



tained in the juices under these conditions, and this tends to destroy 

 the less resistant bacteria, encourage the more resistant, and at least 

 maintain, for a longer or a shorter time, the virulence of a culture 

 freshly isolated from a lesion or cultivated by animal passage. 



THE AVENUE OF INFECTION AND TISSUE SUSCEPTIBILITY 



Successful infection of the body by certain bacteria can be accom- 

 plished only when invasion takes place through appropriate avenues. 

 Thus typhoid, cholera, and dysentery infection seems to take place 

 through the gastro-intestinal tract, and doubtfully by inhalation, and 

 not at all through the skin or urogenital system; gonococci usually 

 enter the body through the genital organs or the eye, and not through 

 the respiratory apparatus or through the skin. The route of infection 

 is less important with microorganisms characterized by great aggres- 

 siveness and producing general, rather than local, infections. For 

 example, in most animals anthrax is a general bacteremia, regardless 

 of the route of invasion; plague rapidly becomes a bacteremia, whether 

 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 



