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induces a certain change, therefore, is that it is the cause 

 in that particular instance. The possibility of other 

 organisms giving rise to the same changes, or effect, or 

 chemical products, must be conceded, and the demonstra- 

 tion of the relations of an organism to such a change rests 

 with the proof that it is a cause. 



Just as there are organisms which induce changes 

 in dead animal or vegetable matter, there are others which 

 are capable of inducing similar changes in living animals 

 and plants, thus living at the expense and frequently to 

 the detriment of the host. The infectious diseases in man, 

 animals and plants, possess as an essential characteristic 

 the property of transmissibility. They are the result, first, 

 of infection that is the entrance of a specific micro organ- 

 ism, and second, of intoxication due to the poisonous pro- 

 ducts elaborated by the microorganism. Poisonous chem- 

 ical compounds may produce the symptoms and the 

 changes observed in an infectious disease. They are 

 the cause of those symptoms and changes, but they 

 are not the cause of disease, since the symptoms and 

 changes thus obtained are not transmissible from one 

 individual to another. Chemical substances have no 

 power of multiplication and the effect observed is, there- 

 fore, directly proportional to the amount of the chemical 

 compound introduced. Microorganisms, however, have the 

 power of multiplication, and the introduction of a minute 

 amount, even a single cell, may bring about entirely dis- 

 proportionate 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 microorganism 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. 



