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_ 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 _secondary_ 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 diphtheriæ_. 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 _Bacterium 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_ 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 tubercle bacilli 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. The characteristic of a pathogenic microörganism 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, 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 susceptible, the infection is severe.