INTRODUCTION 229 



roundabout— as through a special tick in Texas fever, a 

 mosquito in malaria, etc. — or by direct personal contact, as 

 generally in venereal diseases. After all, though exactness 

 is necessary, it is better to learn all possible about the means 

 of transmission of diseases than quibble as to the terms to 

 be used. 



An infectious disease may be acute or chronic. An acute 

 infection is one which runs for a relatively short time and is 

 "self-limited," so-called, i. e., the organisms cease to mani- 

 fest their presence after a time. In some acute infections 

 the time is very short— German measles usually runs five 

 or six days. Typhoid fever may continue eight to ten 

 weeks, sometimes longer, yet it is an acute infectious dis- 

 ease. It is not so much the time as the fact of self-limita- 

 tion that characterizes acute infections. 



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. Syphihs 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 Corynebacterium dipMJierioB , lockjaw due 

 to Clostridium tetajii, Texas fever due to the Piroplasma 

 bigeminum, etc. It is non-specific when it may be due to a 

 variety of organisms, as enteritis (generally), bronchopneu- 

 monia, 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. 



