It is known that many of the germs which cause diseases cannot get into the body unless the protecting membranes have first been injured in some way. Thus the germs that cause plague and lockjaw find their way into the system principally through abrasions of the skin. Many physicians have come to believe that the typhoid fever germ cannot get into the body from the intestine where it is taken with our food or drink unless the walls of the intestine have been injured in some way.
— Insects and Diseases
by Rennie W. Doane