The kitchen and bathroom share a wall. The drain beneath the kitchen sink (double sink; one side with disposal and dishwasher) goes into the wall it shares with the shower. When we run the sink (especially with the disposal on), the filthy water comes up from the drain in the tub.
The tub drains mostly fine, though it has had it's problems in the past. But the dishwasher doesn't clean as well as it should, and I wonder if it's backing up inside.
I have tried running a hand-crank snake down the tub, but this hasn't helped much except to make the tub drain better.