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 manifest 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 disease. It is not so much the time as the fact of self-limitation
that characterizes acute infections.
In chronic infections there is little or no evidence of limitation 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 number of organisms introduced and the point of their introduction 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 diphtheria due to the Corynebacterium diphtheriæ, lockjaw due to Clostridium tetani, 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), 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.