Went out to care for my hens as usual this morning, and noticed my Coraline was sprawled out in a fluffy lump and did not respond to me as usual, just looked up at me rather than jumping up for treats. Picked her up, and she seemed normal (clear eyes, good color skin, comb, no wounds or injuries, etc.), except she was panting slightly and seemed to have slightly thicker than normal saliva hanging in her open mouth. It was clear, not bubbly or frothy, and not at all an excessively thick or viscus mucus. I opened her beak and wiped it up on a tissue I had in my pocket, and offered her a drink of the fresh water I had brought out but she was uninterested, but not in any way distressed. The same amount of saliva I had just wiped away was once more in her in mouth so I decided maybe she had a chicken "runny nose" and carried her back up to the house intending to keep her in an enclosure in the basement so she could recuperate in peace. I had just set her in her enclosure when she began thrashing about, and immediately reached back in to get her out thinking she was uncomfortable. Instead she died in my arms. From discovery to death was less than ten minutes. I am now worried that whatever killed her might be infectious to her remaining 6 sisters. I have sterilized everything of theirs I can and changed their litter, but if it was infectious they are undoubtedly now exposed. They are a completely closed flock, no new birds for years, hardware cloth screens on everything to keep out other birds, varmints and such. No recent change in feed, no moldy straw and fresh water twice a day, or more depending on how cold it gets. I do keep a quartet of ducks but they are not kept near to each other, their supplies are kept separate, and the ducks have also been kept separated from other critters by hardware cloth. These are the only poultry that I ever interact with so it couldn't have come from another flock. I don't think it would be from too little ventilation as this is the same coop she's been living in her whole life without issue. Coraline was old, we can't remember if she would be 9 or 10 years old this spring, and most things I can find online about chicken disease symptoms refer to decreased laying or soft shelled eggs, when I don't think she's laid an egg in a years. Yesterday she was her normal shelf, no coughing, sneezing, lack of appetite, diarrhea, or other symptoms I often see listed. Could it have been just an old chicken's time to go, or does anybody recognize these symptoms (or lack thereof), of something contagious? Just wanted some kind of input so I'm not sitting around waiting for more chickens to die if I could be doing more for them. Its hard when you've kept a friend for almost a decade and then she dies without you being able to help her.