Have you removed the carpet yet? Anything growing nicely under there, like mold or mildew? You will want to be particularly alert for these and rotting of your plywood. If that are signs at all, replace the subfloor too. It should not be a big deal with the carpet up.
If the subfloor is nice and dry and looks to be in good shape otherwise, paint on a coat of 2 of BIN Primer/Sealer from Zinnser. The shellac will block any odors nicely.