I agree with Scott, Nosema is the silent killer and does a lot more damage to colonies than people realize.  I didn't think that we had much of a problem, as we live in a hot climate, but I guess Nosema caused around $30,000 in lost production before I found out what was going on.  Nosema does not cause dysentery, but dysentery causes nosema. Frequent culling of old combs, changing bottom boards in early spring, treating  stored supers with acetic acid and giving the bees fumidil b will help to keep the problem at bay.  Probably every colony in the world is infected and usually it's not a big problem, but when conditions are favourable it can do a lot of damage.  

When we export queens, we have to supply our vet with a bee sample from the attendant bees, he will not issue a health certificate if the bees show ANY spores.  The test is easy to do and we have started doing our own testing to monitor the situation as fumidil b is an expensive item.

Roger White
Superbee Cyprus.