I had her out the first week for errands after her first doc's appointment, and we've been going out a couple times a week since then (she's 7 weeks old now). We went to a big family gathering when she was a month, and to visit friends out of town when she was five weeks. Church every week, too. I'll be honest--I think you're overprotective and borrowing trouble if you're assuming people are going to be sick, but that's just where I'm coming from. If it's what you feel is best, that's fair--because you're the mom you make the call. But I would not expect people to get behind your decision because, frankly, I'm a FTM and I think you're overreacting.
Now, if you heard that Aunt Jennie was sick and all her kids had the flu, I could see bowing out of Thanksgiving. At the same time, why the heck are people going to Thanksgiving if they're sick anyway? And why plan for the worst case scenario?