HYGIENE 543 



These are often considered by those residing in the endemic area 

 to be some of the unavoidable ills to which man is heir. 



(3) Epidemic. 



A disease is said to be epidemic when the cases are widely prevalent 

 over a larger or smaller area. Epidemic diseases, as a rule, tend to 

 spread rapidly and to attack fresh communities. 



Examples are small-pox, \'ellow fever, cholera, plague, dengue, 

 trvpanosomiasis, relapsing fever, anthrax, &c., though some of these 

 are limited in their distribution by the prevalence of their carriers, 

 e.g., the mosquito, glossina\ rat and rat flea, bugs, &c. 



(?) Pandemic. 



A disease is said to be pandemic when the cases become diffused 

 over most parts of the world, as influenza from Russia and plague 

 from India. 



The study of the modes of the transmission of communicable 

 diseases increases much with the opening of new means of intercourse 

 between countries and continents, e.g., the Suez Canal, the Trans- 

 Siberian Railw^ay, the Panama Canal, &c. For example, plague may 

 be brought from Guayacjuil on the Pacific Coast, where it is endemic, 

 and introduced via the Panama Canal into countries where it has been 

 obliterated, e.g., England. The arrival of one plague-infected rat in 

 Liverpool from Japan caused a consternation in that cit}' only a few^ 

 months ago. There were eight cases of plague brought to London in 

 October, 1919. 



When studying the question one has to consider the carriers and 

 conditions under which the disease thrives. For example, if there are 

 no mosquitoes (Stegomyia calopus) in the country of arrival, one need 

 not be over-anxious about importing a case of yellow fever, just as 

 one does not hesitate to send to Europe cases of trypanosomiasis from 

 infected areas to places where there are not any Glossin^e. 



There are many other factors to be considered as locality, climate, 

 season, surrounding's, age and sex of those exposed. 



The spread of ankylostomiasis needs heat and moisture; cold will 

 cut off the epidemic at once and define its margins. Likewise, bil- 

 harzial disease requires molluscs; no molluscs, no bilharzial disease. 

 The old theories of air dissemination are in the main exploded, they 

 being but a cloak for our ignorance, as the old term " eczema " was 

 for skin diseases in general. 



The whole facts affecting the spread of these diseases are not yet 

 ^\ell understood even in advanced countries. We do not know why 

 scarlet fever should tend to become epidemic at intervals of five years 

 in temperate climates where it has become established. 



The same applies to measles, which tends to be epidemic every two 



