CHAPTER XV. 

 INFECTION AND IMMUNITY. 



When parasitic symbionts are fairly large they are 

 said to infest the host; when very small, to infect it. 

 Between infestation and infection no sharp distinction 

 can be drawn, though it is probably more correct usage 

 to employ the term " infection" for the invasion of the 

 body by microparasites of any kind. Any object upon 

 which such organisms may be brought into the body is 

 said to be infective. The body into which they are 

 brought is said to be infected, the organisms through 

 which the infection is brought about are said to be 

 infectious, Infection, being a form of parasitism, 

 implies injury of the host by the microparasites. 



Infecting organisms are, therefore, always pathogenic, 

 or capable of exciting anatomical or physiological dis- 

 turbances. The injurious quality of the organism is 

 characterized as its virulence, and depends upon con- 

 ditions attending its metabolism. Thus, it may liber- 

 ate enzymes, it may produce toxic proteins, it may 

 transform the chemical reaction of the surrounding 

 media, or it may facilitate its own invasive powers by 

 giving off certain offensive substances (aggressins) by 

 which the defensive reactions of the host are inhibited 

 in action. 



The almost universal prevalence of bacteria deter- 

 mines that no higher organism escapes contact with 

 them. Through how wide a range their power of in- 

 vading the tissues of other living things may extend is 

 difficult to answer; it is doubtless very wide, and 

 includes both plants and animals. 



364 



