Originally Posted by Messie

Is it a general damp type of smell or a more yucky something's rotten type of smell. I've had both and a helpful plumber explained things to me.
If it's a damp smell, there's a leak somewhere that needs to be found and fixed. If it's a rotten smell it's more often than not something blocking the exit of dirty water. It doesn't have to be really dirty water, just from the shower is called dirty water. I was advised to try a strong drain/plughole clearing stuff first before calling in the experts or ripping up floorboard. I used good old fashioned soda chrystals rather than an expensive brand of stuff, and sure enough the blockage was cleared and smell went away.

Got to be the cheapest first option for trying to fix it?