Recently installed bathroom in basement. Broke up the floor and installed new piping into existing sewer line that goes directly out to City Sewers as well as the vent pipe that we hooked into the existing vent for the upstairs toilet. For several weeks we had the toilet piping roughed in, and had a "rag" stuck in the pipe to make sure no gas came into the house. 3 weeks ago we installed a brand new AQUADIS DECO KIT toilet. The toilet works great. Flushes properly and haven't had any problems to date. Toilet is not used very often. Maybe once every couple of weeks. Today it rained, and the smell coming from the bathroom is almost unbearable. We flushed the toilet and the smell originally got worse, but now every time we flush it it the smell seems to be getting less and less. There are NO water leaks at all in or around the toilet.

Any suggestions as to what would cause the smell in the first place? Is there a requirement to flush a toilet on a regular basis?