A bathroom, even when it is dirty, should never smell like raw sewage. Sewer gasses are not healthy for humans to breathe and can contain flammable methane gasses, putting everyone in the house at risk of a sudden explosion. If you are unable to determine the cause of the smell, contact a qualified plumber for assistance.
Some people do not clean their toilet for months at a time, and while the toilet does not smell pleasant, it does not smell like raw sewage either. You may have what appears to be a spotless toilet bowl, and yet a raw sewage smell emits from it each time you flush the toilet. Sewer organisms can travel into the bowl, living below the rim where the smell goes back into the air every time you flush the toilet. Since regular toilet cleanings do not necessarily eliminate the problem, you will need to turn to pouring possibly several cups of bleach down the toilet tank's overflow tube.
The various plumbing fixtures in your bathroom all have traps or curved pipes that are just below the initial drain opening, including the toilet. The traps are designed to trap water inside, creating a barrier to sewage smells that otherwise would travel through the pipes and out the drain opening. Traps will dry out when the plumbing fixture is not used often enough and the water evaporates or when a pet drinks out of the toilet bowl. A blocked vent pipe can also cause the water level in a fixture's trap to drop although the likelihood of a blocked vent pipe is not high.
The toilet hooks up to a large drainpipe, called a soil pipe, which leads to the sewer. To keep the sewer gasses out, the toilet not only has a trap but also a two-part seal between the bottom of the toilet and the bathroom floor. An improperly installed wax ring, the first part of the seal, will allow sewer gasses through. A toilet that is not anchored down tightly, or a toilet that can move at all, will eventually break the wax ring or the flange piece that makes up the other part of the seal.
A clog in a sink, shower or bathtub drain can create a smell that very closely resembles a sewer. Hair, make-up, soap scum and other materials that wash down a drain can clump up in the drainpipe, creating a partial or full obstruction in the pipe that you must remove to be rid of the smell. Some clogs will come out when you remove the drain cover or stopper while others you can reach with needle-nose pliers or a bent wire hanger. Deeper clogs require you to use a plunger or a drain snake.