Microorganisms in Relation to Man 571 



influenza, grippe, cholera, lockjaw or tetanus, anthrax, whooping 

 cough, and one variety of dysentery, generally known as summer 

 complaint or diarrhoea of infants. 



This list of diseases is sufficient to emphasize the necessity of 

 knowing the life history and habits of pathogenic bacteria. 

 This knowledge is necessary in order to secure such control 

 of them as will prevent or cure the diseases caused by them. 

 The need of this knowledge is emphasized also by the well- 

 known bacterial infection of plants and the lower animals, to 

 which is due an annual loss of several million dollars. What we 

 need to know is briefly: (1) in what conditions bacteria best 

 develop ; (2) how they multiply and at what rate ; (3) what are 

 their products and the effects of these products upon the 

 organism in which they are evolved ; (4) and how bacteria are 

 transferred from one place to another. The knowledge of 

 these items leads directly to the discovery of the ways and means 

 of making conditions unfavorable for their growth and multi- 

 plication, of destroying or neutralizing their products in the 

 human body, and of preventing their transfer from an infected to 

 a healthy individual. 



As stated above, only a few species of bacteria are pathogenic, 

 or cause diseases in plants and animals. Very many kinds are 

 included in the group of bacteria causing decay and putrefaction ; 

 but there are many others which are entirely harmless, and still 

 others which are indispensable in various ways. The human 

 body is literally alive with many kinds of bacteria, the skin and 

 hair, the mucous membranes of eyes, nostrils, mouth, throat, 

 and the alimentary canal, having each its peculiar kind of bacteria. 

 In normally healthy individuals pathogenic forms may be found, 

 but these seem for the time powerless to cause disease ; only 

 when bodily conditions are favorable do they begin their un- 

 desirable activities. 



285. Useful bacteria. The useful varieties of bacteria are 

 many. They are indispensable in agriculture, in brewing, in 

 the manufacture of vinegar, wines, and other liquors, and of 



