68 INFECTIOUS AND PARASITIC DISEASES. 



over, a person may resist the action of a pathogenic 

 agent at one time and fall victim to it at another. 



INFECTIOUS AGENTS IN DISEASE. 



From the view-point of infectious agents there are 

 five conditions to be fulfilled before they can provoke 

 disease : 



(i) They must be present in sufficient numbers. 



(2) They must reach (what is for them) their portal 

 of entry. 



(3) They must be virulent. 



(4) In the case of certain pathogenic agents, disease 

 can result only when the agent is accompanied by one 

 or more other micro-organisms, or when accompanied 

 by a foreign body. 



(5) If a disease is communicated exclusively by 

 an insect, obviously the presence of that insect is an 

 essential condition. 



Contrary to a wide-spread belief, there is 

 Number, no disease which a single infectious agent, 



acting alone, can produce; that is to say, 

 one micro-organism of a certain species, however viru- 

 lent it may be, is incapable of provoking disease. 

 Experimentally it has been found that even in animals 

 very susceptible to them, it takes large numbers of a 

 particular micro-organism to bring about characteristic 

 results. In diseases occurring among persons we have 

 no way of determining the number of germs that pri- 



