INTRODUCTION TO THE STUDY OF INFECTIOUS DISEASES 377 



subsequent chapters, a great many infectious agents may live sapro- 

 phytic lives in and upon the bodies of human beings, who themselves 

 are not suffering from the disease. Such individuals are known as 

 carriers, and the carrier problem has infused difficulties into sanitary 

 procedure which, in some cases, it is almost impossible to combat. 

 Thus, every community has in it a definite percentage of typhoid 

 and paratyphoid carriers; many individuals carry meningococci, 

 diphtheria bacilli and virulent pneumococci and streptococci; there 

 are quite surely carriers of poliomyelitis and scarlet fever, and it 

 is not impossible that the carrier state can exist for a number of 

 other diseases in which we have not yet been able to prove the 

 condition by actual experiment. 



In addition to the case and the carrier, a source of great danger 

 are unrecognized mild cases. Typhoid fever may take a very mild 

 form, especially in vaccinated people; the atypical cases of 

 poliomyelitis which occur in the course of every epidemic and may 

 occur in interepidemic periods may not be recognized until secondary 

 cases occur; and in many adults who possess a relatively high im- 

 munity, diphtheria infection of the throat may be so mild that no 

 suspicion of the disease is aroused. It is in connection with such 

 occurrences that the diagnostic acumen of the practicing physician 

 is especially important, and it is in the recognition of the atypical 

 cases and in the early diagnosis of the ordinary cases, that the 

 practitioner represents the first line of defense against epidemic 

 outbreaks. 



It is for this part of the protective campaign that we need 

 reporting systems, so that early cases may be centrally collected and 

 charted. Organizations for epidemiological survey to trace the early 

 cases to their sources, and laboratory units to affirm the diagnosis, 

 search out the possible carriers and trace the infection, if possible, 

 to contaminated food or water. Here, too, are necessary arrange- 

 ments for isolation and hospitalization which involve the bac- 

 teriological control by which may be determined when it is safe to 

 release the patient for free association with his fellows. 



The transmission of infectious disease may be either by direct 

 contact from person to person, by indirect contact through materials 

 that have passed from the sick or the carrier to the new victim 

 by food and water and by conveyance through the agency of insects. 

 These factors will be considered in detail in connection with every 

 individual disease, and it is quite clear that, in order to properly 



