A sewer smell coming from a toilet is an indicator something is wrong in the basic plumbing system, and more particularly, the drain system, according to HowStuffWorks. If the toilet is one that no one uses regularly, the problem is likely due to evaporation of water in the toilet trap. Flushing at least once per week keeps the trap full, preventing sewer gas from escaping the drain.
Toilets sometimes develop small leaks that allow water to slowly drip from the trap. If there is water around the base of the toilet for no apparent reason along with the smell, a crack is likely the cause. In this case, the best solution is replacing the broken toilet.
For traps to work correctly, the roof vent for the fixture allows sewer gas to escape into the atmosphere. If birds build nests in these pipes or the snow level on the roof covers them, the gas builds up enough pressure to force its way through the water that seals the trap. It is important for homeowners who choose to inspect the vents to use caution on the roof, especially if there is snow or ice on top of the house.
Cracks in vent pipes and improper installation provide another route for sewer gas to enter the bathroom.