CHAPTER IX. 



§ ]. PATHOGENIC BACTEEIA. 



Introduction. It is surprising that so much misconception should 

 exist as to the functions of bacteria. To the minds of the vast 

 majority of even educated people the term "bacteria" is synonymous 

 with "small organisms that breed disease.'' They overlook, or rather 

 are not aware of the fact, that the number of bacterial species which 

 are pathogenic is small in comparison with the large number which 

 not only do good but fulfil functions upon which our very existence 

 depends. There are, however, out of the total of nearly two thousand 

 known species a few, some thirty species in all, which, from the purely 

 human standpoint, must be regarded as scourges, though possibly if 

 considered from the wider standpoint of the economy of nature they 

 would not be looked on in this light. In this chapter we propose 

 to deal with the broad facts connected with the activities of this class 

 of bacteria. 



All pathogenic bacteria cause a disturbance in the human system, 

 if they are able to multiply. The disturbance is caused by poisons 

 secreted by these bacteria, and this secretion does not take place 

 except when the bacteria are multiplying. Hence the mere presence 

 of pathogenic bacteria is not in itself harmful. Thus, many millions of 

 cholera germs might be present in the body without any harm accruing 

 to the individual : the harm commences only when these begin to 

 multiply. The poisonous secretions of these bacteria are called toxins, 

 and all diseases due to bacteria may be regarded as poisoning processes. 

 An inoculation of a toxin produces the same effect on an animal, as 

 is produced by the inoculation of the bacteria which produce that 

 toxin. The toxin-inoculation is however more rapid in its effects, 

 as is to be expected, for it takes some time for the bacteria to produce 

 the poison in quantity. Such being the case, we can readily under- 



