Your home is shielded from the sewer gasses inside of your sewer lines by p-traps at every sink and appliance. A p-trap is the bend in the pipe that holds water, thus creating a seal blocking these gasses. If one of your p-traps were to fail, you could have a very unpleasant smell in your home, and exposure to sewer gas for a long period of time can be harmful to your health. What might cause a p-trap to fail? One reason is a pipe that isn’t in use very often. In this case, the water will evaporate over time causing the seal to fail. A solution is either to manually run the water in sinks that don’t see much use or you could install a tap-primer – a device that monitors the water level in your p-trap and will add water when needed.
If you have more than 2 pipes that are draining through the same p-trap, the p-trap could be overloaded and fail as well.