FACTORS OF PATHOGENICITY AND INFECTION 233 



harmful effects, while the same bacteria rubbed into the skin may 

 give rise to a severe reaction. 



Animals and man are protected against invasion by bacteria in 

 various ways. Externally the body is guarded by its coverings of 

 skin and mucous membranes. When these are healthy and undis- 

 turbed, microorganisms are usually held at bay. While this is true 

 in a general way bacteria may in occasional cases pass through 

 uninjured skin and mucosa. Thus the Austrian Plague Commission 

 found that guinea-pigs could be infected when plague bacilli were 

 rubbed into the shaven skin, and there can hardly be much doubt 

 of the fact that tubercle bacilli may occasionally pass through the 

 intestinal mucosa into the lymphatics without causing local lesions. 



Even after bacteria of a pathogenic species, in large numbers 

 and of adequate virulence, have passed through a locally undefended 

 area in the skin or mucosa of an animal or a human being by a 

 path most favorably adapted to them, it is by no means certain 

 that an infection will take place. The bodies of animals and of 

 man have, as we shall see, at their disposal certain general, systemic 

 weapons of defense, both in the blood serum and the cellular ele- 

 ments of blood and tissues which, if normally vigorous and active, 

 will usually overcome a certain number of the invading bacteria. 

 If these defenses are abnormally depressed, or the invading micro- 

 organisms are disproportionately virulent or plentiful, infection 

 takes place. 



Bacteria, after gaining an entrance to the body, may give rise 

 merely to local inflammation, necrosis, and abscess formation. They 

 may, on the other hand, from the local lesion, gain entrance into the 

 lymphatics and blood-vessels and be carried freely into the circula- 

 tion, where, if they survive, the resulting condition is known as 

 bacteremia or septicemia. Carried by the blood to other parts of 

 fhe body, they may, under favorable circumstances, gain foothold in 

 various organs and give rise to secondary foci of inflammation, necro- 

 sis, and abscess formation. Such a condition is known as pyemia. 

 The disease processes arising as the result of bacterial invasion may 

 depend wholly or in part upon the mechanical injury produced by 

 the process of inflammation, the disturbance of function caused by 

 the presence of the bacteria in the capillaries and tissue spaces, and 

 the absorption of the necrotic products resulting from the reaction 

 between the body cells and the microorganisms. To a large extent, 

 however, infectious diseases are characterized by the symptoms result- 



