Mucous membranes are barriers against colonization and growth of microorganisms.

Mucous membranes are barriers against colonization and growth of microorganisms. However, mucous membranes, for example in the mouth and gut, are colonized with a variety of different microorganisms, some of which are potential pathogens. Explain how these potential pathogens are controlled under normal circumstances. Then describe at least one set of circumstances that might encourage pathogenicity.