INFECTION AND TRANSMISSION 17 



line of the cranium produces a specific effect on the motor nerves, 

 causing paralysis, presumably through the action of salivary 

 secretions or of the excretion from the coxal glands; the amebse 

 of pyorrhea, or the bacteria associated with them, which infect 

 the teeth and gums give rise to such symptoms as rheumatic 

 pains in the joints, anemia and a disturbance of digestion. In 

 fact it may be said that a very large number of diseases or ab- 

 normal conditions which were once attributed to purely physical 

 causes, such as imperfections in the organization of the body, 

 or which have been accepted merely as common derangements 

 of the human machine for which no direct cause could be found, 

 have been traced to the effect of particular parasites located, 

 perhaps, in some unsuspected part of the body. We are daily 

 widening the scope of this phase of pathology, and this is one of 

 the main reasons for the present important position of parasi- 

 tology among the medical sciences. 



Modes of Infection and Transmission. — The portals of entry 

 and means of transmission of parasites is a question of the most 

 vital importance from the standpoint of preventive medicine. 

 In the past few decades wonderful strides in our knowledge along 

 these hues have been made, but there is much yet to be found 

 out. 



With a very few exceptions animal parasites do not exist in 

 air and dust as do many vegetable parasites, although some 

 spirochsetes, coughed from the lungs or throat, may infect other 

 individuals by being breathed in, and the granules formed by 

 some of these spirochsetes may be blown about with dust and 

 thus infect in the manner of many bacteria. 



Many parasites may be spread by direct or indirect contact 

 with infected parts, e.g., the spirochsetes of syphilis and yaws, 

 the mouth amebse, the parasites of Oriental sore, itch mites and, 

 of course, free-moving external parasites. The parasites of the 

 digestive system and of other internal organs gain entrance in 

 one of two ways. They may bore directly through the skin as 

 larvse, e.g., hookworm. More commonly they enter the mouth 

 as cysts or eggs, e.g., dysentery amebse and Ascaris; as larvae, 

 e.g., pin worm; or as adults, e.g., leeches. Access to the mouth 

 is gained in many different ways, but chiefly with impure water, 

 with unwashed vegetables fertilized with " night soil," or with 

 food contaminated by dust, flies or unclean hands. The para- 



