From research I found 3 different symptoms for Marek's - paralysis, or blindness, or internal tumors and the tumor kind is where you don't know the chicken is ill until it drops over dead without hardly any symptoms until the very end. And there is no way prior to death to identify Marek's in a carrier. It's unfortunate that only a necropsy identifies death-by-Marek's for certain. It was frustrating to read about the way Marek's travels from bird to bird because it is nearly impossible even with biosecurity to keep it from a yard. It travels as you say from the microscopic dander from the chicken's skin that floats on the loose feathers of the chickens or microscopic dander just off of human's clothing/shoes. Just the wind alone can carry this dander for miles and all the biosecurity in the world will not keep it from a yard/flock. I mean we can do our best by not visiting places that have birds or vice versa but we can only do what we can do and pray for the best. Wildlife birds are a concern - I love them but not around my yard.