I love this idea and think it could be a great mechanic... Like in the 1st season, second episode of Walking Dead, they had rub themselves down with a nasty scent (from a corpse) to be able to walk around the zombies in close proximity. I think that bad odor should have its positives and negatives. The more odor you get, the closer to zombies you can get- you smell like them. But, this odor can wash off in the rain, lakes etc. If you are filthy and nasty and get scratched in a zombie fight... you need to take a bath and clean up to reduce the risk of infection and enable your ability to heal. Then by taking a bath, the zombies can now smell the "soap" or antibacterial etc. making you more noticeable. I like the idea of them walking slowly to you and the closer they get the more aggressive, faster and ravenous they become. Also, while "underground" - if that even happens, you need to be clean to cook food or be around food supplies to reduce possible infection