86 INFECTIOUS AND PARASITIC DISEASES. 



these, protozoa, the lowest form of animal life, are of 

 prime importance. Then there are flukes, worms, 

 and insects. Of the pathogenic protozoa known, 

 nearly all depend upon suctorial insects for transmission 

 from one person to another. The role of insects in 

 the production of disease is large, forming a highly 

 important branch of medicine; and it is destined to 

 become much larger. Two kinds of transmission of 

 pathogenic micro-organisms by living agents are recog- 

 nized, (i) in which the insect is merely an accidental 

 carrier of the micro-organism; (2) in which the insect 

 acts as host for the micro-organism during one phase 

 of its life-cycle. Where the insect is an accidental 

 carrier of the microbe, the surface of its body, or its 

 excrement, has been contaminated by feeding upon 

 infectious material. Under such circumstances a 

 disease is transmitted either through the soiling of a 

 wound, or through the contamination of an article of 

 diet. While living upon the insect no increase in 

 numbers of the pathogenic agent takes place. Bac- 

 terial (and possibly a few protozoan) diseases are fre- 

 quently transmitted in this way. In a previous chapter 

 the wide-spread prevalence of typhoid fever in the 

 United States military camps during the Spanish- 

 American war was mentioned to illustrate the trans- 

 mission of bacteria by flies. Besides flies, ants, bed- 

 bugs, fleas, and ticks, may also accidentally carry 

 bacteria, and besides typhoid fever, insects are believed 



