INFECTION. 47 



Endemic and Epidemic Occurrence of Infectious Dis- 

 eases. Most infectious diseases attack human beings with 

 varying frequency. We speak of the sporadic occurrence 

 of a disease when only isolated cases appear ; and of 

 endemic distribution, when a larger number of cases are 

 constantly present. Measles, scarlet fever, diphtheria, and 

 tuberculosis are, for instance, endemic in middle European 

 populations, while cerebrospinal meningitis, mumps, etc., 

 occur only sporadically. Asiatic cholera prevails endemic- 

 ally in the East Indies. 



Under special conditions, any infectious disease may 

 acquire extraordinary distribution, attacking a much larger 

 proportion of the population than before, or extending 

 beyond the borders of its previous area of distribution, and 

 invading neighboring countries. We speak of epidemics 

 of typhoid fever and of diphtheria when the usual number 

 of cases occurring on the average within a given district is 

 considerably exceeded. At certain intervals cholera leaves 

 its Indian home to invade in widespread epidemics {pan- 

 demics) almost the entire inhabited world. The systematic 

 causes that govern the occurrence and the subsidence of 

 such epidemics have been made the subject of investigation, 

 especially by Pettenkofer and by Koch. The epidemic 

 distribution of infectious diseases may be, in large part, 

 explained by the same influences that are responsible for 

 the isolated infection ; but in every epidemic the special 

 biologic relations of the causative agents, as well as the 

 varying predispositions of human beings, demand particular 

 study. Thus, pandemics of influenza are readily explicable 

 from the fact that, on the one hand, the bacilli are contained 

 in the sputum of the sick and with this are carried through 

 the air, and, on the other hand, the susceptibility of human 

 beings to this infection is unusually great. For the com- 

 prehension of epidemics of cholera it is important to know 

 that the bacilli are contained in the dejections, and that 

 with these they frequently find their way upon linen, cloth- 

 ing, contaminated articles of food, etc. ; that they are 

 conveyed by insects, may be transported with soiled mer- 

 chandise, etc. In this manner extension of the disease may 

 take place from case to case in a continuous chain. Explo- 

 sive distribution of a disease, simultaneous invasion of a 

 large part of a community, occurs through equable distri- 

 bution of the infective material over an extensive area as 



