ABSTRACT
Before a microbe or parasite can invade the host and cause infection, it must first attach to and penetrate the surface epithelial layers of the body. Organisms gain entrance into the body by active or passive means. For example, they might burrow through the skin, be ingested in food, be inhaled into the respiratory tract, enter via the genitourinary tract, or penetrate through an open wound. In practice, most microbes take advantage of the fact that we have to breathe and eat, and therefore enter the body through the respiratory and gastrointestinal tracts. Whatever their point of entry, they have to pass across physical barriers such as the dead layers of the skin or living epithelial cell layers which line the cavities in contact with the exterior—the respiratory, genitourinary, and gastrointestinal tracts. These are the main routes of entry of microbes into the body.
