The body has three main lines of defence against disease. These involve mechanical barriers, chemical barriers and cells. Mechanical barriers Although many bacteria live on the surface of the skin, the outer layer of the epidermis seems to act as a barrier that stops them getting into the body. But if the skin is cut or damaged, the bacteria may get into the deeper tissues and cause infection. Hairs in the nose help to filter out bacteria that are breathed in. However, if the air is breathed in through the mouth, this defence is by-passed. Chemical barriers The acid conditions in the stomach destroy most of the bacteria that may…