Simply put, the gas isn't healthy. If the sewer isn't properly vented, you'll get the gas. This is a 'common occurance' where people finish a basement off and put in a toilet w/o venting it. It's things like this. The best solution is to have 2 or 3 plumber make a visit to your site to see where the problem might lie. Then, if you or anyone feel 'ill', ctc your family physician making sure to mention the gas leak.