SOURCES OF INFECTION 83 



tivity on the part of the microorganism until the vital resistance of the 

 host is diminished or the pathogenicity of- the microorganism is increased, 

 when the neutral relations are disturbed and infection occurs. 



RELATION OF INFECTION TO IMMUNITY 



From what has been said, it is apparent that the subject of infection 

 forms the basis for the study of immunology, for, paradoxic as it would 

 at first appear to be, infection must usually have occurred in order that 

 immunity may be acquired. This relation is not always apparent; for 

 instance, man and some of the lower animals may possess a natural 

 immunity td a certain parasite because of the presence of various physi- 

 cal or non-specific defensive factors, or to specific antibodies produced 

 as the result of an earlier and unrecognized infection, or even one that 

 has been inherited; under any circumstances, however, natural immu- 

 nity is usually relative and seldom absolute. In passive immunity the 

 same conditions are generally operative, and the antibodies present in 

 the serum used to confer a passive immunity are produced in some other 

 animal as the result of an active infection. 



It may be stated, therefore, that specific antibodies are produced 

 only by stimulation of the body-cells, and that this stimulation is fur- 

 nished by the infecting agent either in living, disease-producing form, or 

 in a modified and attenuated state, i. e., in the form of a vaccine; thus 

 it will be seen that infection and immunity are intimately associated, 

 and that, 'generally speaking, there can be no pronounced protection 

 unless infection has taken place. 



SOURCES OF INFECTION 



Bacteria are to be found everywhere. For general purposes they 

 may be roughly divided into two classes saprophytes and parasites. 

 The saprophytes are those bacteria which thrive best in dead organic 

 matter, and perform the very important function of reducing, by their 

 physiologic activities, highly organized material into those simple 

 chemical substances that may again be utilized by the plants in their 

 constructive processes, and in this manner maintain the important 

 chemical relation between the animal and the plant kingdom. Para- 

 sites, on the other hand, find the most favorable conditions for growth 

 and activity upon the living tissues of higher forms of animal life. They 

 include most of the so-called pathogenic or disease-producing bacteria. 



No marked separation between these two divisions can be made, 

 as numerous species occupy a transition point between the two. The 



