PARASITISM AND PATHOGENESIS 2OI 



all. In studying each disease it will be necessary to consider 

 the avenues by which the parasite leaves the patient, its existence 

 in the external world and the means of gaining access to its new 

 victim. 



Healthy Carriers of Infection. A person or animal may 

 harbor virulent infectious agents without showing symptoms of 

 disease, and may serve as a source of infection to others. This 

 was clearly recognized in the sixteenth century by Fracastorius 

 as a factor in the spread of syphilis. Only recently has its 

 importance in other diseases been emphasized. 



