Is the shower a dedicated shower, or a tub with a shower? If a dedicated shower, is the floor (pan) tiled, or a manufactured pan? MOre often than it should, a tiled shower has the liner (the actual waterproofing) installed flat on the floor by the plumber, then the sloped floor is installed then the tile by the tiler. Since the tile is not waterproof, eventually there is a buildup in the flat pan since it can't easily flow to the drain (or the weep holes) because that waterproof layer is flat...gravity can't draw it to the drain. This means that stagnent water eventually accumulates. Mortar is naturally basic (pH higher than 7). Eventually, it will be neutralized and then continue to become acidic by the soap, pee, etc. that might migrate down there. Once that happens, things can start to grow in the stagnent water trapped in the pan. At that point, assuming the shower is used regularly, the only way to resolve it is to tear it out and rebuild correctly. A properly built shower pan won't leak or smell like a swamp. An improperly built one might not leak, but can easily start to smell. It can take years to neutralize the cement so stuff starts to smell, but moisture accumulation starts almost immediately. If you live in the desert where the relative humidity is really low most of the year, you may never notice this.