208 GENERAL BIOLOGY OF MICRO-ORGANISMS 



and yellow fever by miasm. All of these are doubtless infectious 

 diseases but some of them are not naturally spread by contact at 

 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 im- 

 portance in other diseases been emphasized. 



