MODE OF SPREAD OF INFECTIVE DISEASES. 729 



1. The Sources of Infection. 



At the present time we only reckon as infective All infective 

 diseases those affections which are caused by the caused by* 6 

 penetration of a pathogenic agent from without into ^^ ly ^ eh 

 the body of the patient, and its multiplication there, the body of the 

 Hence a relatively insignificant quantity of the virus is P ' 

 always sufficient for the infection, but a certain time 

 usually elapses before the development of the action, this 

 time being necessary for the multiplication of the virus. 



If a noxious agent must be employed in a certain 

 and large dose, if it does not multiply in the body 

 of the patient, and if the action sets in relatively 

 quickly, the process is comprehended under the term 

 " intoxication." And in like manner when disease is pro- 

 duced by a so-called miasma, that term implying a 

 gaseous chemical substance or a mixture of unorganised 

 substances not capable of reproduction in the body, we 

 regard it not as infection but as intoxication. 



From the fact that reproduction is necessary, it follows Hence the 

 that the infective agents are all organised beings. It agent? areW 

 has been further ascertained by the researches of the & organisms. 

 last ten years that these organisms quite apart from 

 the animal parasites chiefly belong to the class of bac- 

 teria ; it is possible, however, that the exciting agents 

 of certain human infective diseases belong also to other 

 as yet imperfectly known classes of micro-organisms, 

 e.g., the mycetozoa. 



It follows also, from the reproduction of the infective All infective 

 agents in the body of the patient, that all true infective 



diseases can be transmitted continuously from sick to from the sick 



i 1,1 .,..,, , ., , , to the healthy. 



healthy individuals, although it is possible that for 

 reasons to be discussed later this transmission is at 

 times accomplished only under marked difficulties, and 

 at certain stages of the disease, and in one particular 

 way. Quite recently the malarial virus has been trans- 

 mitted to healthy individuals by inoculation of the blood 

 of sick people, and hence the infective character of this 

 disease has been demonstrated with certainty. 



Although this power of multiplication and trans- 



