INTRODUCTION 215 



In chronic infections there is little or no evidence of limi- 

 tation of the progress of the disease which may continue for 

 years. Tuberculosis is usually chronic. Leprosy in man is 

 practically always so. Glanders in horses is most commonly 

 chronic; in mules and in man it is more apt to be acute. 



Many infections begin acutely and later change to the 

 chronic type. Syphilis in man is a good illustration. 



The differences between acute and chronic infections are 

 partly due to the nature of the organism, partly to the num- 

 ber of organisms introduced and the point of their intro- 

 duction and partly to the resistance of the animal infected. 



An infectious disease is said to be specific when one kind 

 of organism is responsible for its manifestations — as diph- 

 theria due to the Bacterium diphtherioe, lockjaw due to 

 Bacillus tetani, Texas fever due to the Piroplasvia bigeminum, 

 etc. It- is non-specific when it may be due to a variety of 

 organisms, as enteritis (generally), bronchopneumonia, wound 

 infections. 



Henle, as . early as 1840, stated certain principles that 

 must be established before a given organism can be accepted 

 as the cause of a specific disease. These were afterward 

 restated by Koch, and have come to be known as "Koch's 

 postulates." They may be stated as follows: 



1. The given organism must be found in all cases of the 

 disease in question. 



2. No other organism must be found in all cases. 



3. The organism must, when obtained in pure culture, 

 reproduce the disease in susceptible animals. 



4. It must be recovered from such animals in pure cul- 

 ture and this culture likewise reproduce the disease. 



These postulates have not been fully met with reference 

 to any disease, but the principles embodied have been applied 

 as far as possible in all those infections which we recognize 

 as specific, and whose causative agent is accepted. In many 

 recognized infectious and contagious diseases no organism 

 has been found which is regarded as the specific cause. In 

 some of these the organism appears to be too small to be 

 seen with the highest powers of the microscope, hence they 

 are called " ultramicroscopic" organisms. Because these 

 agents pass through the finest bacterial filters, they are also 



