Mold growth occurs in moist environments and more-so especially in porous environments. Materials such as glass, metal and plastic generally do not support active colonization of mold, howevever materials such as wood, carpet, textiles and sheetrock do. Mold growth in sheetrock is perhaps the first sign a home or business owner will have to alert them to a mold infestation. By that time, it's generally safe to say that the sheetrock should be removed completely, as mold will have inhibited the material throughout (from one side to the other) as sheetrock is very porous and acts like a sponge wicking up any surrounding moisture. If sheetrock is covered by wallpaper, the space in between the wallpaper and the sheetrock can also be a breeding ground for mold. This is an are of the home that many forget about as being a trouble area. Again, the porous nature of materials such as paper and sheetrock are a perfect medium for mold growth and must not ever be ignored!