Many of these bacteria are harmless, some are protective, producing by their growth such changes in the surface fluids that these become hostile to the existence of other and pathogenic forms.

The surfaces also frequently harbor pathogenic organisms which await some condition to arise which will permit them to effect entrance into the tissues.
The surfaces of the body protect from invasion to a greater or less degree.

The skin protects by the impervious horny layer on the outside, the external cells of which are dead and constantly being thrown off.

Bacteria are always found on and in this layer, but the conditions for growth here are not very favorable and the surface is constantly cleansed by desquamation.

The new cells to supply the loss are produced in the deepest layer of the epidermis, and the movement of cells and fluids takes place from within outwards.