The air in a sewer pipe contains methane gas and is naturally, "smelly". This is why each fixture must have a p-trap. The p-trap is designed to "trap" some water, thus keeping the sewer gas from entering your home (see image below). This includes your sinks, toilets, tub / showers, floor drains and toilets - all fixtures. Note: although Methane gas is flammable, it is not necessarily harmful to your health.
The most common cause of sewer gas entering your home is a dry floor drain or p-trap. When floor drains are initially installed, they are required to have a "primer valve" on them. This valve is connected to your cold water supply and emits water intermittently to keep your floor drain’s p-trap "primed" or full of water. What happens, especially in older homes, is that the "primer," either quits working or is disabled by someone during a remodel or toilet installation (in older homes primers are actually connected to the toilet tank).
Without a steady flow of water to keep the p-trap "primed" or full, sewer gas will enter the environment and stink to high heavens! The other cause could be a fixture that hasn’t been used in a very long time and the water has evaporated from the p-trap.
SOLUTION: Either have a new primer valve installed or run some water into the dry fixture.