216 INTRODUCTION 



frequently called "filterable." The term "virus" or "filter- 

 able virus" is likewise applied to these "ultra-microscopic" 

 and "filterable" agents. 



The term primary 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 secondary generalized tuberculosis, or 

 primary typhoid fever by a secondary pneumonia. 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 Bacterium diphtherial. 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 harmless 

 condition gives rise to an infectious process. If the Bacillus 

 coli normal to the intestine escapes into the peritoneal cavity, 

 or passes into the bladder, a severe peritonitis or cystitis, 

 respectively, is apt to result. "Boils" and "pimples" are 

 frequently autoinfections. Such infections are also spoken 

 of as endogenous to distinguish them from those due to 

 the entrance of organisms from without — exogenous infec- 

 tions. Relapses are usually instances of autoinf action. 



Those types of secondary infection where the infecting 

 agent is transferred from one disease focus to another or 

 several other points and sets up the infection there are 

 sometimes called metastases. Such are the transfer of 

 tubercle bacilli from lung to intestine, spleen, etc., the for- 

 mation of abscesses in internal organs following a primary 

 surface abscess, the appearance of glanders nodules through- 

 out various organs following pulmonary glanders, etc. 



The characteristic of a pathogenic microorganism which 

 indicates its ability to cause disease is called its virulence. 

 If slightly virulent, the effect is slight, if highly virulent, the 

 effect is severe, may be fatal. 



On the other hand, the characteristic of the host which 

 indicates its capacity for infection is called susceptibility. 



