FUNCTIONS OF THE PARASITIC BACTERIA 39 



hosts and usually excite detrimental changes in those hosts. 

 It is to the parasitic group that the pathogenic or disease- 

 exciting bacteria belong. 



Strictly speaking none of the pathogenic bacteria with 

 which we are acquainted are obligate parasites, that is, 

 none of them grow and multiply only in the body of a living 

 host; for all have been cultivated under artificial conditions 

 on dead, nutrient cultural materials. They are nevertheless 

 properly classified as parasites for it is only under conditions 

 of parasitism that they exhibit those activities that make 

 them the objects of special interest. 



When circumstances admit of the various members of 

 this group getting access to the living hosts in which they 

 find conditions favorable to their growth and multiplica- 

 tion there results the state known as "disease." In some 

 cases the disease is local, i. e., it involves only the tissues in 

 the immediate vicinity of the invading bacteria; in others 

 it is general; involves the entire body and eventuates in 

 the death of the host. 



As we study the peculiarities of the disease-producing 

 bacteria more closely we find that in inducing disease they 

 do not all operate in the same way, though the ultimate 

 forces used by them in the destruction of living tissue are 

 throughout analogous, i. e., they are poisons. 



In some cases the parasite finds the circulating fluids of 

 the host the most favorable place for its growth and develop- 

 ment. Under such circumstances it is not uncommon for 

 the blood and lymph vessels of an infected animal to be 

 almost filled with the parasites within a short time after 

 the invasion. To such a state the designation "septicemia" 

 is given, that is, there is a septic condition of the blood, 

 "blood poisoning" as it is commonly called. 



