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.      

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jk but i am serious about the part that most people cant answer it beacuse you did not mention the section you are talking about. but it would if you added some detail and were a bit more specific.