One of my year old Leghorn chickens died today. She stopped laying 2 months ago, at the same time that some of the hens went through a molt, so I didn't worry. I had noticed she had a very poopy butt about 2 weeks ago and a droopy comb, so started adding probiotic yogurt to the feed, but she took a turn for the worse 3 days ago and stopped eating, and became lethargic. She finally died today. I have 3 other hens and they all seem well, but before I replace her should I be worried for the other hens? Her poop was a loose but not bloody. I could not see any other outer signs of illness, apart from the droopy pale comb. Advice?