Many organisms, although their growth in the stomach is inhibited, are not destroyed there and pass into the intestines, where the conditions for infection are more favorable.
This large and very irregular surface is bathed in fluid which is a good culture medium and but a single layer of cells covers it.

The organisms which cause many of the infectious diseases in both man and animals find entrance by means of the alimentary canal, as cholera, dysentery, typhoid fever, chicken cholera, hog cholera.
Infection by the genito-urinary surface is comparatively rare.

The surface openings are usually closed, and the discharge of urine has a mechanical cleansing effect.

The wide tube of the vagina is further protected by a normal bacterial flora which produces conditions hostile to other and pathogenic bacteria.

The most common infections are the sexual diseases, which are due to organisms which find favorable conditions for growth in and on the surface and which are conveyed from a similar surface by sexual contact.
It remains a question whether bacteria can penetrate an intact surface producing no injury at the point of entrance and be carried by the lymph or blood into internal organs where they produce disease.
Internal infections are often found with seemingly intact body surfaces, but it is impossible to exclude the presence of minute or microscopic surface injuries by which the organisms may have entered.
It is also possible that a slight injury at the point of entrance may heal so completely as to leave no trace.
The chief danger from wounds is that their surfaces may become infected.