Chickenpox,
Hang in there. You can't always know about these things. My guess is that the hen did have a respiratory illness at some point and recovered and is a carrier. She could have also been perfectly healthy and picked up a bug at the show. Try not to be hard on yourself. You are not the first that brought a bird into your flock that might have been sick and for sure you will not be the last. So sorry you are losing your chicks. That's truly hard!
Hang in there. You can't always know about these things. My guess is that the hen did have a respiratory illness at some point and recovered and is a carrier. She could have also been perfectly healthy and picked up a bug at the show. Try not to be hard on yourself. You are not the first that brought a bird into your flock that might have been sick and for sure you will not be the last. So sorry you are losing your chicks. That's truly hard!