My mother used to have a formula for stuff she used to take off some of the varnish when she didn't actually want to refinish, just wanted to take off the worse stained or darkened part of the finish. But I don't have it written down anywhere. It was three equal parts of linseed oil, turpentine, and something else, can't remember what. Something she learned about from a museum somewhere. But like Mrrzy said, wood has enough tannins and such that it is pretty good at killing surface bacteria or whatever over a little time. I would try running a moist cloth over it to pick up any dirt, but don't soak it.

SRS