Cold and cough are common health problems that affect children the most. If not treated properly, it can build up mucus in the lungs. Mucus is a normal, slippery and stringy fluid substance produced by many lining tissues in the body. It is essential for body function and acts as a protective and moisturizing layer to keep critical organs from drying out. Mucus also acts as a trap for irritants like dust smoke or bacteria. A running or stuffed up nose is a pain, but that extra mucus helps your body stay healthy. Mucus is an important substance. The body produces to protect itself from viruses and bacteria.