BACTERIAL ACTIVITY 173 



which are developed by bacterial growth, independent of 

 the medium in which grown, and have even been obtained 

 in cultures in proteid-free media. Of the ptomaines, 

 putrescin and cadaverin are extremely poisonous, and most 

 cases of meat-poisoning, cheese-poisoning, and vegetable- 

 poisoning are due to one or another of these ptomaines. 



INFECTION. 



The invasion of the animal body by bacteria is spoken 

 of as infection if it gives rise to disease. The definition 

 requires extension to cover the case of diphtheria, where 

 the invasion by the micro-organism is often very slight, 

 but where the disease is due to the invasion of the body by 

 the toxins or bacterial products. In most cases the 

 infection is due to both the bacteria and their products, 

 in varying degrees. 



In the first place it is useful to note that the skin and 

 the mucous membranes of the alimentary tract, the mouth, 

 the nasal passages, the upper respiratory tract, the con- 

 junctivae, and the genital passages, are normally inhabited 

 by various species of bacteria. Some of these are 

 facultative parasites, and seize the opportunity of a break 

 in the surface, or other injury, to grow and multiply, and 

 so produce disease. Others are pure saprophytes, non- 

 pathogenic in any circumstances to the body on which 

 they harbour. 



The definition of the infective diseases will be useful here. 



An infective disease, or rather a specific infective disease, 

 is one which results from the introduction into the body, 

 (i) by wounds, (2) by the air-passages, or (3) by the 

 alimentary tract, of a definite ferment, or poison, or 

 micro-organism, which grows and multiplies in the body. 



In some of these diseases the poison is given off again, 

 and they are then spoken of as infectious, or transmissible 

 from person to person. Where contact is necessary for 

 transmission, they are called contagious. The tendency 

 is to give up the use of these terms, infectious and con- 

 tagious, and simply to speak of infective diseases, which 

 are transmissible in various ways, as by the air, by food, 

 by contact, by fomites, by insects. 



