We had a hen die from what I believe was Egg Yolk Peritonitis. When we noticed she was looking ill, we separated her from the flock in a large dog crate, she died within hours. I went to check on her and her legs were paralyzed, I picked her up and her head fell to the side, spilling out some yellow liquid and she just... died. She had not laid an egg in a few weeks but I had never heard of Egg Yolk Peritonitis and she was prone to going broody so I just assumed that is what it was. She showed no signs of being ill until it was too late to help her. We initially suspected Avian Flu but they do not have contact with any other birds and the other two hens and rooster that had constant contact with her have showed no signs of illness. However, not knowing exactly what killed her we separated the flock for observation. We put our 3 youngest (would have been 1 year in July) in a crate together, they have been in one many times together and spend every day together. They were hatched together (sadly by the hen who had just passed, although they were not her eggs) and have never been apart. 2 roosters and 1 hen. Within hours of being put in the crate they were all dead. There were no signs of trauma and I know they were not fighting. It was cloudy and rainy so I know it wasn't heat stroke. They all looked like they simply laid down, tucked in and went to sleep... never to wake up. Their combs had turned dark purple but other than that nothing was different about them. I just do not understand what happened to them and after raising them, it really hurt to find them dead. They were good chickens, knew their names, came when called, ate out of our hands and were very easy to handle. It just kills me that we may have done something to cause their deaths. As I stated we have 3 left, 2 hens and a rooster that seem completely fine so far so I am at a loss. If anyone has any ideas on what it possibly could have been please comment below. Thank you.