This is just one of those things that people don't tell you about and almost nobody talks about, because they don't know about it. I learned about this back when I was doing Y2K work on a major natural gas pipeline that stretches a couple thousand miles.
It starts off with this: How do you locate a leak in a natural gas pipeline? You look for the pile of dead flies.
Why is this? It's because Methane gas is what decomposition and rotting gives off. This attracts flies because they're looking for food. Natural gas is Methane gas, (which came from decomposition deep in the Earth) and it's all methane is methane as far as a fly is concerned. So they all flock to it. BUT because there's no oxygen in the area of the leak they're all clustering around, they suffocate. Hence the pile of dead flies.
Now, under every sink, shower, and tub, and built into every toilet, you have a 'u-joint'. The purpose of this is to keep from venting your cesspool/septic system/sewer/whatever into your house. Mostly because it smells bad, but also because it draws flies (third point is that it cuts down on the spread of disease, countries that don't use these - *china*cough*cough* - have disease and pandemic issues).
How does this apply to you? Well, do you have a sink or a tub you never (or rarely) use? Have you noticed an increase in flies in that room? Maybe you have a bar sink in the house or out on the back patio?
If you don't use it periodically, the water in the u-joint will evaporate and 'swamp gas' or methane is going to come up out of the pipes. It'll stink, and it'll draw flies. I once owned a house that had a caretakers apartment built into the barn. It was always packed with flies and dead flies. Turns out the idiot who put in the shower didn't put a u-joint under it, so I had to tear out the concrete and install one. Oh, and clean up ten years worth of dead flies in the insulation :-6
So, to avoid these problems in those 'unused' facilities, a couple of times a year, run the water in that shower or tub or sink. Flush that toilet. You'll cut down on flies and on stink, and yes, even disease.
This has been a public service announcement from the people who don't like flies, stink, or disease!
(Oh, and your house will have vents into the system via your drainage pipes that come out the roof - don't cover those, or you'll have all sorts of other unenjoyable issues!)