244 president's address — section i. 



One would like to believe that infectious bacteria perish with 

 considerable speed under ordinary conditions. Our comparative 

 freedom from disease points either to that or to our possession of a- 

 considerable amount of protection against them. Our immunity 

 probably largely depends upon the fact that we do not seek infection, 

 but rather avoid it as much as possible. It is, however, difficult to 

 avoid being infected by certain throat affections, such as the common 

 cold, which is conveyed from person to person by means of spray 

 particles in the air. Bacteria have been found in localities which clearly 

 indicate this method of distribution. Kinyoun (17) examined the 

 dust and scrapings of railway carriages for pathogenic bacteria, and 

 found positive results in 27 per cent, of the tests. The microbes were 

 chiefly staphylococci and pneumococci, but two of the tests contained 

 Bac. diphtheriw. In a carriage which had been used for conveying^ 

 consumptives to a sanatorium, Bac. tuberculosis was detected. It is- 

 one thing, however, to detect injurious microbes in such places and 

 quite another thing to show that they have other than the most remote 

 chance of becoming converted from a potential into an actual danger. 

 It has never been shown that the men who clean railway carriages- 

 contract lung affections more frequently than other people. The 

 detection of staphylococci points to nothing, for one has only to intro- 

 duce a fragment of garden soil under the skin, perhaps accidentally 

 by means of a wound, to bring about a formation of pus teeming with 

 these microbes. 



We know that the soil is the ultimate destination of the majority 

 of bacteria, and in it the less injurious forms, which we speak of in a 

 general way as soil bacteria, are able to vegetate and multiply. The 

 noxious kinds find the soil to be an inhospitable medium, and in it they 

 rapidly perish. But if the soil is polluted extensively, and that means 

 the accompanying addition of nutritive material in which the microbes 

 are suspended, the pathogenic forms may gain the upper hand, so to 

 speak, and an infectious or dangerous soil is the result. Dust from 

 such a soil may go far to produce an epidemic, as was painfully ex- 

 perienced in the typhoid outbreaks during the South African campaign. 

 The question of infectious dust is, therefore, closely connected with the 

 viability of pathogenic bacteria in the soil. It appears that if the 

 infection of the soil has not been too great the pathogenic microbes 

 rapidly die out ; not, however, on account of the scarcity of food, but 

 from the poisonous action of the products of the putrefactive bacteria 

 which are better adapted to the environment. Sidney Martin (19) 

 was able to detect typhoid bacteria a year and a half after they had 

 been put into sterilised soil, but in ordinary unsterilised garden soil 

 he could not recover them after the second day. 



Dust from ordinary soil may, therefore, be accepted as bemg 

 innocuous so far as the typhoid and probably many other non-sporu- 

 lating pathogenic bacteria are concerned, unless the infection has been 

 recent, or the pollution has been great. 



One might imagine that every particle of dust floating in the air 

 has upon its surface or within its crevices one or more bacteria. Such, 



