INTRODUCTION 231 



less condition gives rise to an infectious process. If the 

 Escherichia coli normal to the intestine escapes into the peri- 

 toneal 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 infections. Relapses are usually instances of 

 autoinfection. 



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 tuber- 

 cle bacilU from lung to intestine, spleen, etc., the formation 

 of abscesses in internal organs following a primary surface 

 abscess, the appearance of glanders nodules throughout 

 various organs following pulmonary glanders, etc. 



Focal Infection.— A focal infection is an infection usually 

 small and circumscribed from which toxins, sometimes 

 organisms, are absorbed and carried to other parts of the 

 body where they give rise to s^^mptoms. The focal infection 

 itself may show slight symptoms or none at all. The effects 

 in other organs are frequently not associated with the "focus." 

 The commonest "foci" are at the roots of the teeth, the tonsils, 

 sinuses about the head, chronic appendicitis, prostate, 

 Fallopian tubes. The organism causing focal infections is 

 most frequently the streptococcus. Neuritis, "rheumatism," 

 rheumatic and muscular pains, headaches, "dyspepsia" are 

 frequently associated with focal infections. 



The characteristic of a pathogenic microorganism which 

 indicates its ability to cause disease is called its mrulence. 

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

 effect is severe and may be fatal. 



On the other hand the characteristic of the host which 

 indicates its capacity for infection is called susceptibility. 

 If slightly susceptible, infection is slight; if highly suscep- 

 tible the infection is severe. 



Evidently the degree of infection is dependent in large 

 measure on the relation between the virulence of the invad- 



