What?! There was a first?
@It_is_I_Rae : there’s various strains of Mareks that affect each flock and chicken a little different. The most unfortunate fact is that in nearly every situation, Mareks is watch and wait. You may, or may not, lose another chicken quickly. You may, or may not, lose your whole flock. You may, or may not lost only a couple of birds.
if you have Mareks or suspect it the number one thing you can do is keep your birds healthy and clean and stress free, and cross your fingers you do not have a virulent strain. Many people can and do keep Mareks-positive flocks with success. Many, many very esteemed poultry vets believe it is SO ubiquitous that it exists in some form in nearly every backyard flock.
this is not to mean that there are not times where Mareks is devastating. It’s just to mean there are times where it is manageable.
@Josip, the cull a whole flock advice is really reserved for either the most virulent strains (but still not always) and commercial flocks and major breeders.