36 



to the way in winch they are transmitted from one indiviilual to another. 

 It is perliaps needless to say that diseases are carried from one individual 

 to another, from host to host, much after the fashion of weeds carried 

 from one lield to another. The seed of a weed may jiain access to a field 

 by being blown in by the wind, or it may have been brought in by an ani- 

 mal, especially by birds ; many weeds have been brought in by impure 

 garden seeds. Cheat or chess among wheat means that the seed was 

 present ; it does not mean the transformation of one species into another, 

 nor does it mean a spontaneous generation. 



The railways are important factors in the distribution of weeds, as 

 they are of diseases. Before the days of railways new diseases traveled 

 slowly, cholera and influenza required a long time to encircle the globe in 

 their early migrations ; today diseases may si)read rapidly. In a thinly 

 settled country, weeds and diseases spread slowly, while the massing of 

 people in cities, especially in the absence of sanitation, favors dissemina- 

 tion. 



Diseases due to specific causes can be grouped in various ways, like 

 weeds ; whether native or foreign ; whether coming to stay, or to disappear 

 after a short time ; whether spreading rapidly and then dying out, or 

 spreading slowly but surely and permanently, etc. Looked at in this light 

 we might regard Milk Sickness as a native disease whicli is disappearing;' 

 Cholera as a disease which has come in repeatedly but on account of un- 

 favorable conditions never gained a permanent foothold ; Malaria as 

 spreading rapidly and lasting for a long time and then declining; Tuber- 

 culosis as coming in and spreading slowly but surely and not yet having 

 reached its maximum among us. Measles, scarlet fever, smallpox, whoop- 

 ing cough, etc., need only be referred to. 



. Classificatioint of Diseases Accoruing to Their Modes of Transmis- 

 sion : In a general way we may classify diseases according to how they 

 are carried from one individual to another thus: 



1. By direct contact — from one host to another. 



2. Transmited through insects. (Notably malaria.) 

 o. Diseases conveyed by or through food. 



-i. AVater-borne diseases. 



5. Air and dust-borne diseases and affections (notably tuberculosis 

 and pneumonia, with a host of other respiratory affections and a variety 

 of aches and pains and functional disturbances.) 



