How can I fix a smelly sink drain?

We went away for a week and when we came home, our kitchen sink drain SMELLS almost mildewed/moldy...(that's what I get for going on vaca and having a good time!) I already did the baking soda vinegar trick, and I ran hot water - I don't have a disposal, and I do have septic. But it's the kitchen sink drain that smells, nothing else.