odor

I smell sewer gas every other day or so in my basement bathroom which has a sink, toilet, and shower. The p-trap on all of them are full and when I get down and smell the drain of the shower, it has a slight stench of the sewer. Is something wrong? How might the gas be getting into the room if...

We put in a new bathroom sink & faucet about six months ago. Noticed when running cold water there is a "rotten" smell. Replaced the aerator and didn't help; replaced the plastic supply line and smell was gone....for a couple of months. It's back. It's only when the cold water is turned on...