If a home has been built without the aid of vents, then vents, or in place of vents, air emitters should be installed near the p-traps.
A good place to start would be at the lavs and sinks, then tubs and washers, and then the toilets.
Sometimes the air emitters are a nice way to go if they are allowed by the local plumbing inspector. Good ones like the Studor brand are not cheap, but then cutting into walls at this point is a bit costly too.
A. If you look on your roof, you will see pipes sticking out of the roof aprox 12" high. For every pipe that goes down, one needs to go up. The obvious reason we have vents is that sewer gases need to be vented outside of the dwelling. Not so obvious is what happens if they are not included in the waste and vent design. Imagine yourself at McDonalds drinking a soda from a straw. If you put your thumb over the straw, you can pull liquid up from the cup. Remove the thumb and see it instantly drain out! When liquid goes down a pipe, air needs to follow it. Without the vent pipe, the draining liquid will try to suck air through the P-traps on the plumbing fixtures,(tub, sink, etc.) glurp, glurp! If it manages to do so, you may know it from the "smell" coming from the now dry seal on the P-trap. Without vents, draining one fixture may cause another fixture in the house to back up, yuck! A waste and vent system should keep sewer gas out of the dwelling and drain every fixture well.