De Zouche. — Bacteria and their Belation to Disease. 45 



will illustrate some of the peculiarities of the visitation of 

 epidemic disease. 



Questions will arise as to why the disease should be so 

 much more fatal during the middle or height of the epidemic 

 than toward the end. Bearing on this question is the fact 

 that certain bacteria may, under special conditions, develope 

 intense virulency. Pasteur found that the virulence of rabies 

 became greatly intensified by passing the virus through a 

 series of rabbits ; and in swine-plague, if the microbe is inocu- 

 lated into a pigeon and from this passed throiTgli a second 

 pigeon, and from the second to a third, and so on, the microbe 

 became " acclimatised " in the pigeon, " and the blood of the 

 later pigeons in the series proved much more virulent to the 

 pig than even the most infective products from a pig which 

 had died of the so-called spontaneous swine-plague." Other 

 conditions by which the virus may be modified or destroyed 

 are temperature and the admixture of chemical solutions with 

 the nutrient material, while the virus may be altered accord- 

 ing as the bacteria grow in the living bod}-, or out of the body 

 on decomposing matters — that is, as parasites or saprophytes. 

 The bacillus of cholera has been cultivated in gelatine and on 

 potatoes by Koch, and he is of opinion that it can reproduce 

 itself and multiply in decaying animal matters outside the 

 human body. It certainly would appear that the germs of 

 cholera, on being transplanted to a district hitherto unaffected, 

 develope intense virulence. While we are not in a position to 

 give a categorical answer to the question proposed, we see 

 what circumstances might happen to a bacterium by which its 

 virulent properties may be preserved or even intensified. Each 

 specific pathogenic bacterium has its own conditions of tem- 

 perature, moisture, and nutrient substratum, in which it best 

 flourishes and produces disease, and future researches will 

 doubtless determine what those conditions are for each bac- 

 terium outside of the laboratories of investigators. One fact, 

 however, has been well established with regard to zymotic — 

 that is, bacterial — diseases — namely, that they have a pre- 

 ference for individuals whose bodies are in a low state of 

 vitality — a fact known long before the discovery of bacteria. 



I referred at a former part of this address to the supposed 

 influence of the seasons in tlie causation of certain epidemic 

 diseases. This is expressed at the present day by the term 

 " epidemic constitution " of the season or year. One year we 

 find pneumonia prevalent, another year pleurisy, another 

 typhoid. With our present knowledge of bacteriology we may 

 explain the occurrence of particular epidemic constitutions by 

 ■ the fact that specific bacteria happen to find at such times the 

 temperature, moisture, and nutrient material — that is, the 

 human body — under fit conditions for their development ; and 



