Places that are often or always damp can be hard to maintain completely free of mold--some of the most common mold growth areas are bathrooms, crawlspaces and laundry rooms. If there's some mold in the shower or elsewhere in the bathroom that seems to reappear, increasing ventilation (running a fan or opening a window) and cleaning more frequently will usually prevent mold from recurring, or at least keep the mold to a minimum. In a crawlspace, the need is different. It's best to seal the crawlspace completely from outside air, install a dehumidifier, and isolate the crawlspace from the dirt floor with an antimicrobal barrier.