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It sure can make you nervous especially if you never have seen afb in a hive yet, but like Tristan says, there's nothing you can do to stop them getting it, but you can stop spreading it. Maybe you can ask someone from a beekeepers club to come and have a look with you together? And if you find it you'll remember forever what it looks like. Then destroy the hive and get another one as it is a lovely and interesting hobby, you'll never be bored if you have a couple of hives!