There is a terrible septic odor increasing daily in my shower drain for the past week. Prior to this, I have never had an odor from this or any other drain in this five year old house. I do know that the closest entry point to my septic system is this first floor shower. One time a few years ago, the water from the upstairs washing machine backed up and poured out of that shower drain flooding my bathroom. I have read the threads on this topic, but have not seen a proper solution posted that has really worked for anyone. Please help.