258 BACTERIOLOGY. 



teria are under the very best conditions of environment 

 they possess a maximum physiological activity. They 

 may give rise to intense poisons, pronounced pigments, or 

 other characteristic chemical products. When, however, 

 these conditions are altered and the environment becomes 

 unfavorable, the bacteria are likely to respond at once by 

 a corresponding lessening of vitality. The chemical pro- 

 ducts which were formed, when the organism was in a 

 vigorous, healthy condition, are now diminished in amount 

 and may even be replaced 'by others. Attenuation, there- 

 fore, is a phenomenon which is common to all bacteria, 

 although the term was first employed in connection with 

 the loss of virulence of certain pathogenic organisms. 



The above four rules have been fully complied with in 

 a large number of infectious diseases. In others the first 

 two rules are satisfied but the third is not, owing to the 

 difficulty of obtaining a susceptible animal. Again, the 

 first rule may be the only one complied with, as in leprosy, 

 where the isolation of the organism has not, thus far, 

 been unquestionably successful. And again, a large num- 

 ber of infectious diseases remain (such as small-pox, scar- 

 let fever and measles) in which even the presence of a 

 specific organism has not been definitely shown. 



It is evident that while a great deal of information has 

 been gained during the past 15 years in regard to the caus- 

 ation of disease, there still remains a great deal of work 

 to be done. Enough facts have been gathered to clearly 

 demonstrate the importance of the new science of bacteri- 

 ology to medicine, surgery and hygiene. What was once 

 called the "germ theory of disease" has ceased to be a 

 theory since it has been reduced to incontrovertible facts. 

 That micro-organisms are the causes of disease has been 

 demonstrated. 



Although many of the infectious diseases have been 

 shown to be due to bacteria, it must not be forgotten that 



