I get that rotten egg smell as well (amongst other smells). I think it is most likely bacteria in the gut, and my current guess is the 'sulfate reducing bacteria' which turn sulfate/sulfur/sulfur amino acids into hydrogen sulfide. If I take a sulfury supplemenet, such as glutathione, I can smell rotten eggy in a day or so. At this point though I am guessing, since I sense no-one in the medical world knows or bothers about us. It could also be to do with sulfur oxidation/metabolim as someone suggested. The body generates some H2S within itself, but I suspect this load is coming from the gut. Nor unless some other bacteria/fungus/ parasite can generate H2S.