Once again, it's not the wet mattress but the path it took to get there is what is important. To get from the outside to the inside it has to get between the outer skin and the inner wall and along that path, it soaks the insulation and the wood substructure. That is where the issues are.
Just because you fix the leak, the water has already permeated and soaked the insulation and wood so things start growing (mold / mildew) because it cannot dry out. It really only takes one soaking and the mold / mildew starts growing.