I was under the impression that Marek's disease almost always affects immature birds, and rarely adults. Very helpful to find this old thread, confirming Marek's in an older chicken: https://www.backyardchickens.com/t/323724/necropsy-results-in-on-limping-chicken Over the past 1-2 years, I have had several adult chickens develop weakness in leg or wing, or staggering, or circling. Some birds recovered after a few days, one has chronic residual, mild incoordination, and I put another one down because he could no longer eat or drink on his own. Then there was a sudden unexplained death in a previously healthy chicken. But most of the flock is unaffected. The affected ones were all adult chickens; no pullets or cockerels have been affected. Right now I have 7 six-month old birds, all of which look healthy, while 2 of older hens are having problems with leg weakness/staggering. Given that I've had ONLY adult birds affected, could it still be Marek's? What else could it be???