ADDITIONAL NOTES ON CONTAGION. 279 



grees of heat and moisture in the atmosphere are necessary to their production; high 

 degrees of heat always increase the spread, and add to the malignancy, of the disease ; 

 filth, and other qualities of the atmosphere less known, vary their types arid exalt their 

 grades; the constitution of the patient renders him more or less susceptible of particular 

 diseases; the addition of a fermeut (which is found wherever the disease exists) most 

 certainly communicates and characterizes it; and laslly, under certain circumstances of 

 purity and temperature of the atmosphere, it is found almost impossible to propagate 

 those diseases, even by the aid of a ferment; and this observation applies, in some 

 measure, even to small pox, measles, &c. 



The sentiment, therefore, which you express, and so well illustrate, that the contagious 

 diseases of your third class, although they may originate in circumstances of heat, moist- 

 ure, and filth, and some other less evident qualities of the atmosphere, yet are unques- 

 tionably, and with more certainty, produced and propagated by the introduction of a fer- 

 ment characterizing the type and grade of the disease, is, I believe, strictly and literally 

 true ; and upon this opinion only can a well-regulated police and quarantine laws be 

 founded. The question you ask, why A after visiting B, ill of dysentery, plague, yellow 

 fever, Sec. is seized with the identical disease of B, when you consider the universality of 

 the fact, is decisive as to the existence of a peculiar virus, or fomes sui generis, producing 

 that particular form of disease; and the observation of Sydenham, that the prevailing 

 epidemic swallows up all other diseases, confirms it ; and 1 think you treat the noo-con- 

 tagionists with too much lenity, when you say they differ from us only in terms : far from 

 it ; they differ in fact, and most dangerously so ; for, by denying the generation of a pecu- 

 liar ferment in and about the bodies of the sick, and the propagation of contagion from 

 patient to patient, they deny the utility, and are led to the neglect of some of the most im- 

 portant precautions against the introduction, importation, and propagation of such diseases. 



Your first class of contagious diseases is strictly and clearly defined ; they can be 

 communicated by contact only: is not the materies morbi of these diseases always 

 generated within the body; and whether it consists of animalculae, or a chemical mixt, 

 are they to be found anywhere else? 



The fact that the diseases of the second class are communicable at every season of the 

 year, during the heat of summer as well as during the severest cold of winter, in a pure as 

 well as an impure atmosphere, forms the best distinction between this class and the third. 

 But in as far as it is true that none of the second class can be suffered more than once in a 

 lifetime, I am almost led to conclude, that in these, too, the materies morbi can be generated 

 only in the bodies of the sick: t he first origin of these diseases, and their occasional re-appear- 

 ance in places where they had not been ssen for years before, I confess forms a difficulty, 



