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. 4. It must be recovered from such animals in pure culture 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 diseases recognized as infectious and contagious 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 frequently called “_filterable_.” The term “_virus_” or “_filterable virus_” is likewise applied to these “ultramicroscopic” and “filterable” agents.