48 IMMUNOLOGY 



Endemics, Epidemics, Pandemics. — When a few cases of an 

 infectious disease occur from year to year in one locality, even 

 though this is quite large, the disease is said to be endemic in that 

 region. When more than the usual number of cases develop in a 

 community, the phenomenon is called epidemic. The disease may 

 spread to adjacent communities and constitute a large epidemic. 

 When it becomes world-wide in nature, it is called a pandemic. A 

 small percentage of the population at large harbors pathogenic 

 organisms known to cause specific infections. These individuals are 

 called earners and are thought to be responsible for endemic dis- 

 ease and for many epidemics. During pandemics, it has been ob- 

 served that the various diseases spread along the lines of travel, 

 but there seem to be other factors involved that arc at present 

 unknown. 



Incubation Period. — The time interval between infection and 

 the occurrence of symptoms is called the i^icuhation period. This 

 is frequently of importance in the spread of infectious diseases 

 since many are more contagious during the latter part of this period 

 when the patient is unaware of the nature of his infection. The 

 duration of this incubation period is quite variable. In diplitheria 

 it is short, being from twenty-four to forty-eight hours, wliilc in 

 typhoid fever, measles, and smallpox, it is commonly two or moic 

 weeks. 



The converse of infection is freedom from disease. Large areas 

 of population may be free from certain infectious diseases for 

 reasons which may be illustrated as follows : 



1. Environmental, Not Involving- Immunity 



Protection Due to Lack or Contact. — Euru|>can people did 

 not suffer from syphilis prior to the return of Columbus' sailors 

 from the West Indies where syphilis was i)revalent among the 

 natives. That this freedom was due to lack of contact rather than 

 to great morality or a high degree of immunity is evident by the 

 virulence of the infections which developed and the high mortality 

 that prevailed after it was introduced. It might be noted here that 

 this is the general experience when a new disease is introduced into 

 a population. 



