36 



to the way in wliu-li they are tviuismitted from one individual to another. 

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

 to another, from host to host, nun-h after tlio fashion of weeds carried 

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

 hy being blown in hy the wind, or it may have been brought in l>y an ani- 

 mal, ospeci:;lly by l>irds; mauj- weeds have l)een brought in l>y 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 glolje in 

 their early migrations ; today diseases may spread rapidly. In a thinly 

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

 l^eople in cities, especially in the absence of sanitation, favors dissemina- 

 tion. 



Diseases due to specilic causes can [)e grouped in various ways, lilic 

 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 which 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 aeclining; Tuber- 

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

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

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



Classiiicatioa' of Diseases Accoruikg to Their Modes of Tra:nsmis- 

 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 thi-ough insects. (Notably malaria.) 

 o. Diseases conveyed liy or through food. 



4. Water-borne diseases. 



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

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

 of aches and jjaius and functional disturbauces.) 



