RELATION OF BACTERIA TO DISEASE 159 



"V D equals the form of disease (acute, chronic, 

 = local, general, etc.), V equals the virulence of 



R the organism and R equals the resistance of the 



host. D changes as V or R varies. The species, race, indi- 

 vidual peculiarities, age, life conditions, etc., and channels of 

 infection must also be taken into account in explaining varia- 

 tions. 



Determination of pathogenic species. In order to prove 

 that a bacterium is the cause of an infectious disease, Koch 

 insisted that the following conditions should be fulfilled. 

 These are often referred to as Koch's dicta. 



1. The organism must be obtained in pure culture from 

 the organs of the infected individual. 



2. The organism must be cultivated for a number of gen- 

 erations in artificial media. 



3. Healthy animals must be inoculated with a pure cul- 

 ture after passing through several generations and the disease 

 produced. 



4. Pure cultures of the organism must be obtained from 

 the lesions in the inoculated animal. 



These requirements do not apply to all of the disease pro- 

 ducing microorganisms especially the higher bacteria and 

 fungi or to those produced by protozoa. 



Distribution of pathogenic bacteria. A few species of 

 pathogenic bacteria seem to exist normally as simple parasites 

 on certain mucosae of the healthy individual such as are 

 found in the upper air passages of most of the domesticated 

 animals. 1 Others are able to live in the soil, such as the 

 bacilli of tetanus and black leg. The evidence seems to indi- 

 cate that B. tetani has its normal habitat in the intestines of 

 horses, cattle and perhaps other animals. Others appear to 

 exist in infected individuals only. It is necessary to learn 

 the life history of each species of disease producing bacteria 

 its normal habitat, its means of dissemination, the method of 

 its invasion and its resistance to destructive agents, such as 

 sunlight, drying and disinfectants. 



1 Moore, Bulletin No. 3, Bureau of Animal Ind. 1893. 



