INTRODUCTION. 21 



some similar process, may contain no micro-organisms, and 

 may be perfectly fit to drink so long as it is fresh, although, 

 if it were allowed to stand in a warm place and exposed to 

 dust and germs of various kinds, there would soon develop 

 in it such an enormous number of organisms that it would 

 be looked upon as totally unfit for drinking purposes. On 

 the other hand, water containing little or no organic matter 

 might be so contaminated by certain disease germs that it 

 would be absolutely dangerous to health, and even life, if 

 used for domestic purposes. 



The relation of bacteria to sewage need not be insisted 

 upon, as it is evident that with such a large quantity of 

 organic matter, in which putrefactive changes must be rapidly 

 going on, bacteria of many kinds must necessarily appear, 

 and it is easy to conceive that an enormous number of 

 different species might be present, especially where the 

 sewage is diluted as it flows into comparatively pure water. 



Let us observe how a disease-producing organism may 

 find its way into water that is used for drinking or other 

 purposes, and thence may attack a comparatively healthy 

 individual. In typhoid fever, a germ, which will after- 

 wards be described, has been found. It has special charac- 

 teristics, and may be separated as a pure culture. A patient 

 has typhoid fever ; the bacilli which we find specially in the 

 walls of the alimentary tract pass out along with the excreta, 

 and, under ordinary circumstances, would be killed by the 

 addition of disinfectant fluid to the stool ; but, as frequently 

 happens, some of the excreta without the addition of a 

 disinfecting fluid by chance finds its way into the drains, or, 

 worse still, on to the surface of the soil near a well or some 

 other source of water supply. The typhoid bacillus con- 

 tinues to multiply in the water or in the organic matter 

 of the sewage, from which it ultimately finds its way into 

 the water, and although such water may appear pure 

 enough (it is often beautifully clear and sparkling), as soon 

 as it is taken into a slightly disordered stomach and 

 intestinal canal, the bacillus gains a foothold, and another 

 patient is attacked with typhoid fever. This may happen 

 simply through the rinsing out of a milk pail. A patient 

 partaking of milk (a most admirable nutrient medium for 

 the growth of the typhoid bacillus) from such a pail is 

 struck down with " Typhoid." Another example : there 



