138 CHAPTEB 16. 



274. Distinction between inoculation, contagion and infection. 



To ensure the transmission of some diseases a tangible portion of the 

 poisonous matter from a diseased animal requires to be placed on a 

 mucous membrane underneath the skin, or on a fresh wound, or on some 

 delicate absorbing surface. 



This is what is called Inoculation. In this way, for instance, small- 

 pox may be produced in the human subject, or glanders and farcy in 

 horses and men. Under circumstances favorable to its development a 

 very small portion of matter is sufficient to reproduce the original disease. 



Contagion is but a variety, as it were, of inoculation. In either case 

 the poison is transferred from the sick to the sound. In the case of 

 inoculation the passage of the matter is patent and obvious ; whilst in 

 diseases termed contagious it is sufficient that two animals should come 

 in contact. The particles of matter, which pass from one to the other, 

 may be intangible and invisible. The fact that the matter has so passed, 

 though not demonstrable to the eye, is yet proved by the result. 



Infection is a term used to denote the spread of a disease by particles 

 of matter floating in the air and absorbed into the system without actual 

 contact of the sick with the healthy animal. 



No broad line or distinction can be drawn between inoculation, con- 

 tagion, and infection. They glide insensibly one into the other. The 

 difference is one of degree, not of kind ; for in all cases the poisonous 

 matter is carried from the diseased to the sound animal. 



Infection, it will be observed, implies a greater degree of communica- 

 bility than contagion ; and similarly this latter than inoculation. 



Many diseases which, under circumstances favorable to health, such as 

 good ventilation, cleanliness and good condition, can only be reproduced 

 by positive inoculation, may under less favorable circumstances be com- 

 municated by contagion ; and under still less favorable circumstances by 

 infection. On the other hand, diseases which are in their nature highly 

 infectious, often become innocuous and unable to reproduce themselves 

 under good sanitary arrangements. 



275. Distance at wlticli infectious diseases can be communicated. 



The question is often asked, to what distance does the power of infec- 

 tion extend. It is not easy to give a definite answer to this question. 

 The emanations, whether proceeding from the poisonous miasma which 

 originally generated the disease, or from the exhalations or excretions of 

 the bodies of the diseased animals, float through the air much in the 

 same way as the invisible scent of flowers. These emanations may be 

 conveyed a considerable distance in a concentrated and undiluted form 

 under circumstances favorable to their transmission. But, as a general 

 rule, just as scent, when separated from the body which gives it off, soon 

 loses its power, so these morbid emanations in general speedily lose their 

 active qualities and power of evil. A few yards of space, if in a thorough 

 draught, is usually sufficient to ensure such a dilution of the poison as 



