PART ONE 

 PRINCIPLES OF INFECTION, 



CHAPTEK I. 



PARASITISM, IXFECTIOUSNESS, CONTAGIOUSNESS. 



Parasitism is the condition in which a plant, or 

 an animal being, lives on or within another living 

 organism. A true parasite always derives its sus- 

 tenance from the tissues of its host. 



An infectious disease. is one which is caused by infectious 



; """"' ~T~- T -, T Disease. 



living organisms which in some way have entered 

 the bodyfwhere they multiply and liberate poison- 

 ous substances. Accordingly the word has refer- 

 ence to the nature of the cause of the disease. It 

 is from the Latin inficere, meaning to place in or 

 into. 



Some parasites may live on a host without 

 causing appreciable damage; they are non-patho- 

 genic parasites. In this case they may derive 

 their nutrition from some of the excreted non-liv- 

 ing products of the host, living as pure sapro- 

 phytes, 1 or the amount of nutritious substance 

 which they obtain from the host may be so little 

 that the health of the latter is not impaired. This 

 is true of organisms which normally inhabit the 

 intestinal tract. 



1. A saprophyte is defined as a vegetable organism 

 which lives on dead organic matter. An organism which is 

 habitually saprophytic may become pathogenic under the 

 proper conditions (bacillus of malignant edema). And, on 

 the other hand, a pathogenic parasite lives a saprophytic 

 life, when it grows in our artificial culture media. 



