How do I get rid of a strange smell in the kitchen which I suspect is due to the plumbing?

This smell is like a sewer smell and occurs only when someone takes a shower in the upstairs bathroom. It lasts maybe 5-10 minutes and goes away. It's gross and we've asked a plumber about it and he told us we'd have to redo the entire kitchen because the plumbing is not properly vented or something. Any ideas?