Horrible Basement Smells

There is a really foul smell coming from our basement, I think from the shower. It is off and on but seems to happen more when we use the shower. The shower is used frequently and it can get so bad that it comes through the vents to upstairs. Our house is over 50 years old and over the past year we have had 4 plumbers try to figure this out. One suggested pouring bleach down the draing but it doesn't seem to do much good.