I've lived in this older house for about 6 months. There are two bathrooms downstairs, each with a tub and toilet. Several weeks ago a sewer odor began seeping up from the drains in one of the bathrooms (but not the other.) Both bathrooms are vented to a single vent, and I've been told that the problems lies in not having each tub and toilet from each bathroom separately vented. If this is the case how come I have only recently been having this problem? Do I need to add vents, or is there some other (and easier) problem?