Most affairs that take place in a marriage don't happen because one day one of the partners woke up and decided to have an affair. It's something that happens little by little. It's so easy to say, that nothing will ever happen, that it's only a harmless little infatuation, but unfortunately this is the same thing spoken by countless others who eventually did let something happen.
I apologize if this seems harsh, but part of my job is working with marriages, and all too often I've seen damage done because somebody who started with a simple, harmless crush on a co-worker, or friend found them self in a situation that they weren't expecting. "I never thought it would happen to me" or "I never meant for this to happen", are words that are far too common.
Why even put yourself into a situation that has the potential of damaging or ruining your marriage? What benefit is there in allowing feelings like this in your heart for somebody else? Is that temporary thrill or excitement really worth risking your marriage over - and believe me, the risk is there.
Having a crush on some celebrity is one thing, having a crush on someone you know (especially someone who seems to feel the same way about you) is completely different - it's playing with fire.
My advice would be to do whatever it takes to get this person out of your life until those feelings are gone. Whatever changes are necessary do it, no matter how big or how small. And like others have said, do your best to try and let your feelings for your husband replace those feelings for this other person. Remember what it is about your hubby that made you fall in love with him. Find ways to let your husband's love cause this same "pitter-patter" in your heart.
Again, I'm sorry if this seems harsh, but I've seen it happen to too many marriages not to be worried by it.