A(n) infection is a small region of infection from which a pathogen may move to another part of the body to establish a second infection.

A focal infection is a small region of infection, from where the pathogen can move to other areas of the body to cause a secondary infection.      

Explanation:

The focal infection theory is a theory that states that many chronic diseases (such as cancer, arthritis, mental illness) are caused by a focal infection. A focal infection is a small region of infection or a localized infected region, which can lead to a secondary infection at a site, which is susceptible to infections. Examples of such focal regions are nasal sinuses, kidney, gall bladder, liver, etc.

Therefore, a focal infection can be defined as a small region of infection, from where the pathogen can move to other areas of the body to cause a secondary infection.