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, commonjy known as the 

 iour 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 



