The smell of sewage or sewage-like odor in the shower is nothing new. Let me first start with the simpler symptom and solution. Most guys know what I'm talking about, so sorry ladies, but it's true. I've been given the task several times before in our home to unclog the shower drain. I'd rather be watching TV than sticking my finger in the drain stopper and pulling goopy long hair out to clear a slow drain. The grease from body oils, lotions, hair products and soap glued together with long hair gets stuck in the drain then incubates with humidity giving off that welcome aroma. 

If it's still clogged after cleaning with the finger, try Drano Max Gel Clog Remover. If that still doesn't work, rent a plumbing snake that manually sends a flexible cable with a brush-like tip that can clear or pull out any obstruction up to 70 ft. down your pipe. Hair and grease buildup can lodge deep into the line. If the clog has somewhat been removed, pour some vinegar down the line and leave overnight to kill the odor. 

Time to call the plumber.  

If the above hasn't helped, then you probably have other factors.

Showers built with tile usually have a secondary drain pan underneath to catch water in case the grout or caulking cracks. This secondary drain pan is supposed to collect the water and protect the rest of the home from water penetration, then drain down the same drain, but sometimes doesn't. If the water stays in the pan underneath, mold, mildew and stagnant scum collects, thus smell.

Under your shower drain is a P-Trap that looks like the trap under your sink. Access to this piece is difficult takes demolition. It's much harder if the floor is concrete. 

The water in this trap is what keeps the smell of sewage back. If this trap is installed incorrectly, the water will drain out every time, never blocking the sewage gases. It is imperative that this trap keep water. This is why you can sometimes see water down the shower drain, because the trap is full. When a property is empty for long periods of time, this trap dries out, letting the smell in. Improper installation of vent pipes ,like the ones coming out the top of your roof can also cause malfunctions. If you flush the toilet, and your shower drain or sink makes gurgling sounds, your getting back pressure.  It's then time to call a plumber. 

These are a few but not only, symptoms and solutions to the smelly shower syndrome.

Source: Steve Medina – Professional Inspector