Organisms from the mouth can extend into the various large salivary glands by means of the ducts and give rise to infections.

The tonsils, particularly in children, provide a favorable surface for infection.
The mucous surface extends into these forming deep pockets lined with very thin epithelium, and in these debris of all sorts accumulates and provides material favorable for bacterial growth.
The lungs at first sight seem to offer the most favorable surface for infection.

The surface, ninety-seven square yards, is enormous; it is moist, the epithelial covering is so thin as to give practically no mechanical protection, large amounts of air constantly pass in and out, and the surface is in contact with this.

They are protected from infection in many ways.

The tubes or bronchi by which the air passes into and from the lungs are covered with cilia; the surface area of these tubes constantly enlarges as they branch, the sum of the diameters of the small tubes being many times greater than that of the windpipe, and this enlargement by retarding the motion of the air favors the lodgment of particles on the surface whence they are removed by the action of the cilia.