184 INFECTION AND IMMUNITY 



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 favor- 

 ably 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 elements 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 microorganisms are disproportionately virulent or plen-. 

 tiful, 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 circulation, where, if they 

 survive, the resulting condition is known as bacteriemia or septicemia. 

 Carried by the blood to other parts of the body, they may, under favor- 

 able circumstances, gain foothold in various organs and give rise to 

 secondary foci of inflammation, necrosis, 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 dis- 

 turbance of function caused by the presence of the bacteria in the capil- 

 laries and tissue spaces, and the absorption of the necrotic products 

 resulting from the reaction between the body cells and the micro- 

 organisms. To a large extent, however, infectious diseases are char- 

 acterized by the symptoms resulting from the absorption or diffusion 

 of the poisons produced by the bacteria themselves. 



Bacterial Poisons. — It was plain, even to the earliest students of this 

 subject, that mere mechanical capillary obstruction or the absorption 

 of the products of a local inflammation were insuSicient to explain the 

 profound systemic disturbances which accompany many bacterial in- 

 fections. The very nature of bacterial disease, therefore, suggested the 

 presence of poisons. 



It was in his investigations into the nature of these poisons that 



