Why does the sink smell so bad?

Our bathroom sink was like this a couple of months ago and I completely took all of the pipes apart and cleaned the gunk out of them. I then rinsed them in hot water and bleach and rembled the pipes and we have not had that problem since. Even thought the pipes appear clear and the water is able to drain they are probably caked in gunk. That is what is causing the funky smell. I don't even want to tthink of what that gunk was I cleaned out!!!