CHAPTER V 



INFECTION VEGETABLE AND ANIMAL PARASITES 

 THE INFECTIVE PROCESS ANTI-BODIES ANTI- 

 SERA AND ANTITOXINS IMMUNITY 



Infection 



BY the term INFECTION is meant the invasion of the living 

 tissues by living micro-organisms which grow and multiply 

 at the expense of the host. A disease produced by the 

 growth and multiplication of micro-organisms is termed 

 an infective disease, and is transmissible in most instances 

 by inoculation. If the micro-organisms are from time to 

 time discharged from the body of the host, either with 

 the excreta, secretions, desquamated particles, or in some 

 other way, the disease becomes infectious or contagious, 

 according to the ease with which another individual 

 becomes infected, and the material which conveys the 

 infection is often termed the contagion. Thus, in scarlatina 

 and smallpox the contagion is very readily conveyed from 

 person to person even for a distance through the air, and 

 these are infectious diseases. Ringworm and syphilis, 

 as a rule, require more or less close contact for infection 

 to take place, and these are, therefore, contagious diseases ; 

 while malaria is neither infectious nor contagious, since 

 persons in the neighbourhood never directly contract the 

 disease, though it can be conveyed by inoculation, and 

 it is therefore infective only. But the distinction between 

 infectious and contagious is mainly one of degree, and these 



144 



