QUESTION: My shower drain occasionally has a musty smell, even though I keep the shower clean. What causes the smell, and what can I use to treat it?ANSWER: There are many possible causes for the smell.The problem could be water that is standing in an area underneath the drain, said Jenny O'Brien of the Minnesota Department of Labor & Industry. If the odor has a sewage tinge, there might be a cracked or rotted-out pipe in the wall.Other possibilities include an improperly installed drain, a faulty toilet wax ring in the same bathroom, a dead rodent behind the wall of the shower or biofilm that has gathered in the drainage system.If you haven't done so, remove the strainer and use a brush to clean as far down the pipe as possible.If the odor persists, you're best off calling a plumber.