The crazy thing is that I just attended a bee meeting that had an hour long presentation on Nosema, and I'm still asking.
I have a hive that has been strong, by all appearances. Today, I looked at that hive and there were three or four dots of dark bee poop on the back of the hive and two bees that had crawled up on my house and died.
So, I opened up that hive and its just booming. I had two mediums boxes and then I checkerboard two more on top. I saw no bee poop inside the hive. The bottom three mediums are full of bees and brood. The brood area has expanded into the third box. They're storing nectar and pollen in the top medium. In other words, other than the funny looking bee poop, this hive looks great to me. Also, since they are already putting nectar in one of the boxes I will harvest from, I don't really like the idea of treating for Nosema.
So, taking all of this into account, should I not worry about Nosema right now on this hive?