I lived in an apartment in Italy that had the same problem, one bathroom that stunk like a sewer. But it was only bad for a few months of the year, in the summer I think. We talked to other people who lived in different buildings in same city that had similar problems but everyone just shrugged and chalked it up to poor infrastructure. We moved, but even in this house we are in now, the plumbing stinks in the summer. We just close all the drains when not in use. It isn't that bad in this house. It's gross though but people here seem used to it!