Hey there everyone, I have a bit of a mystery I hope you'll help me with. My parents have a small flock of chickens, kept on a grassy enclosure which is moved once every several months. The coop has proper ventilation, and they're fed layer pellets or poultry/fowl crumble. Most of the birds are standard hatchery chickens, vaccinated for Marek's at the hatchery. Four of them had been recently purchased at a show, and these birds were quarantined for three weeks so as to not introduce any foreign diseases to the flock. These newer birds are about six months old. Three weeks ago, one of these chickens began sneezing. It didn't appear to be much, and other than sneezing and slightly congested nostrils, the bird acted normally. It has retained these symptoms, and continues to be lively and eat regularly. Two weeks ago, another of the new chickens began to show trouble standing up. It did a sort of ball-change type move to stand up, and frequently spread out its wings for balance or support. Upon standing up, the bird acted fine. I removed this bird from the pen temporarily to see if it would recover, but it didn't seem to help all that much. Eventually, it became droopier and less active. While the family was away for a week, the pet-sitter reported the bird had died. (This person actually works in a veterinary office, and brought the bird in! The vet didn't see anything they could do, but I'll be inquiring for further details shortly). Just today, I was alerted to the fact that a mature (about two, maybe three years old?) bird in the flock (rooster) seemed to be sneezing and light, generally acting strangely. I checked it out, and also noticed it would occasionally stumble, bracing itself with its wings like the other bird. He also did the standard 'pull-head-in' type behavior that can get anyone a bit nervous. This chicken was vaccinated for Marek's at the hatchery. Upon further investigation, a young cockerel also appears to have trouble standing, but once up acts normally and energetic. He shows no sneezing or respiratory issues. Two old hens are sneezing occasionally, two others appear fine. The pullet who first developed the sneezing still appears congested, but active. Another pullet shows no signs. Altogether, this has befuddled me. I'm not sure if I'm looking at two separate outbreaks, an outbreak and a nutritional deficiency, or something completely different. Because all the older birds have definitely been vaccinated for Marek's, I find it unlikely that that's the culprit. On reading about coccidiosis in-depth, I feel like this may be a possibility. While I'm familiar with poultry and their tendencies, I've never had an outbreak in nine years of raising birds, so I'm attempting to handle this as calmly and efficiently as possible. This flock is managed by my parents, not me, so I may be fuzzy on some of the specifics, otherwise I'd happily answer any questions. If anyone has a solution or suggestion, I'd love to hear! For now, I'm going to try to bring up the protein in their diet, as a protein deficiency is one of my theories. Perhaps the sneezing is separate, and the bird who died had something else entirely? I know it's a long-shot, but possible. Thanks for any help you can offer, and here's a picture of the cockerel while trying to stand. Thanks for your help, and thanks for reading through my long-winded poultry disease adventures.