230 INTRODUCTION 



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 

 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 "ultramicroscojnc'' organisms. Because these 

 agents pass through the finest bacterial filters they are also 

 frequently called ''filterable/' The term ''vims'' or ''filter- 

 able virus" is likewise applied to these " ultramicroscopic" 

 and "filterable" agents. 



The term lorimary infection is sometimes applied to the 

 first manifestation of a disease, either specific or non-specific, 

 while secondary refers to later developments. For example, 

 a secondary general infection may follow a 'primary wound 

 infection, or primary lung tuberculosis be followed by second- 

 ary generalized tuberculosis, or primary typhoid fever by a 

 secondary typhoid pneumonia. The terms primary and 

 secondary are also used where the body is invaded by one 

 kind of an organism and later on by another kind; thus a 

 primary measles may be followed by secondary infection of 

 the middle ear, or a primary influenza may be followed by a 

 secondary pneumonia, or a primary scarlet fever by a second- 

 ary nephritis (inflammation of the kidney). Where several 

 organisms seem to be associated simultaneously in causing 

 the condition then the term mixed infection is used— in 

 severe diphtheria, streptococci are commonly associated with 

 the Corynebacterium diphtherioB. In many cases of hog 

 cholera, mixed infections in the lungs and in the intestines 

 are common. Wound infections are usually mixed. Auto- 

 infection refers to those conditions in which an organism 

 commonly present in or on the body in a latent or harm- 



