INFECTION AND CONTAGION. 137 



selves, become infectious or contagious under less favorable circumstances. 

 The emanations from the sick hang thick and noisome in a close, foul, 

 unchanged atmosphere, and by concentration and ferment acquire a posi- 

 tive power of disturbing health and reproducing disease. 



273. Mode of propagation by infection or contagion. 



We have already stated our belief, that the ordinary cause of a disease 

 running through a stable is simply that the same cause, whether it be 

 miasma arising from bad drainage, or foul air arising from want of venti- 

 lation or from want of cleanliness or over-crowding, or low sanitary con- 

 dition arising from bad or insufficient food, or debility caused by over- 

 work or by neglect or any other such-like cause, affects all the animals 

 placed under the same conditions. 



We do not, however, deny that diseases under circumstances favorable 

 to their propagation may be communicated by sick to healthy animals, 

 independently of the original cause of the disease. For instance a glan- 

 dered horse, bought we will suppose at a distant fair and placed among 

 healthy animals, may communicate the disease to them. Water as a 

 vehicle for the carriage of contagion has been hitherto somewhat over- 

 looked. Public water-troughs need the most scrupulous cleaning by the 

 sanitary authorities, or they may become very serious sources for the 

 spread of contagious diseases. With fair care, however, there need be no 

 fear. Public water-troughs are a great public convenience, but they 

 ought to be in the hands of responsible authorities and not in those of 

 an irresponsible association. 



The mode in which the propagation of disease occurs is believed to be 

 as follows. In certain diseases there are given off from the patient's 

 body, in his breath, in his perspirations and other secretions, minute 

 but invisible particles or microbes. Although not sufficiently tangible 

 to be examined even by the microscope or to be subjected to analysis by 

 the chemist, yet it is tolerably well ascertained that these particles, 

 which form the matter of infection and contagion, contain, like many 

 other animal products, the four organic elements of carbon, hydrogen, 

 oxygen and nitrogen. In common with such bodies they possess under 

 certain circumstances the power of ferment, and, like the yeast plant, 

 have under favorable conditions remarkable powers of reproduction. 



When a sufficient particle of the matter of contagion is either through 

 the lungs or through the skin or otherwise absorbed into a healthy body, 

 it may under circumstances favorable to its development ferment and 

 reproduce itself. This process may occupy, according to the nature of 

 the poison, a few days or several weeks, at the end of which time sym- 

 ptoms of the disease appear. 



Each infectious or contagious disease gives rise to a ferment peculiar to 

 itself, which in due time develops symptoms similar to those of the 

 original disease. 



