258 INFECTION AND IMMUNITY 



present in the alvine discharges of the sick, and which 

 are, consequently, well named filth diseases. 



We now see clearly, however, why in certain par- 

 ticulars relating to its etiology it resembles the mala- 

 rial fevers. It is limited as regards its prevalence 

 to comparatively warm latitudes or to the summer 

 months in more temperate regions and is dependent, 

 to a certain extent, upon rainfall or the proximity of 

 standing water, because these conditions are neces- 

 sary for the propagation of mosquitoes. 



It is evident that in view of our present knowledge 

 relating to the mode of transmission of yellow fever, 

 the preventive measures which have heretofore been 

 considered most important i. e., isolation of the sick, 

 disinfection of clothing and bedding, and municipal 

 sanitation are either of no avail or of comparatively 

 little value. It is true that yellow-fever epidemics 

 have resulted, as a rule, from the introduction to a 

 previously healthy locality of one or more persons 

 suffering from the disease. But we now know that 

 its extension did not depend upon the direct contact 

 of the sick with non-immune individuals and that 

 isolation of the sick from such contact is unnecessary 

 and without avail. On the other hand, complete 

 isolation from the agent which is responsible for the 

 propagation of the disease is all-important. In the 

 absence of a yellow-fever patient from which to draw 



