RELATION OF BACTERIA TO DISEASE. 255 



have the power of multiplication, and the introduction of 

 .a minute amount, even a single cell, may bring about en- 

 tirely disproportionate results. The invading- organism is, 

 therefore, the cause of the disease, since it imparts the 

 characteristic property of transmissibility ', and, through the 

 .action of its chemical products, produces the symptoms 

 and effects of that disease. 



In order to positively demonstrate the causal relation 

 of a micro-organism to a given disease, it is necessary to 

 meet the following requirements, commonly known as the 

 four rules of Koch: 



1. The organism must be present in all cases of that 

 disease. 



2. The organism must be isolated and obtained as an 

 absolutely pure culture. 



3. The pure culture of the organism when introduced 

 into susceptible animals must produce the disease. 



4. In the disease thus produced, the organism must 

 be found distributed the same as in the natural disease. 



To these four requirements, a fifth may be added, 

 namely: That the chemical products of the organism must 

 produce the characteristic symptoms and effects of that 

 disease. 



The demonstration of the constant presence of an or- 

 ganism in a disease is accomplished by making hanging- 

 drop examinations of the fluids or exudates of the body; 

 by making stained cover-glass preparations, or by stain- 

 ing sections of the tissues and organs. Frequently, the 

 direct detection of the organism is difficult, owing either 

 to its scarcity, or to the absence of definite character- 

 istics. In such cases artificial culture, or the animal ex- 

 periment, will usually prove the presence of the organ- 

 ism. In the advanced stages of some diseases the germs 



