Marek’s is everywhere, that is true. However, very few strains express this disease in high numbers. However, some strains do express it more than others, and some not at all. I once had a strain that would express Marek’s 50% of the time. This was a problem. I eventually got rid of that family.
Most farms see this maybe 5% or less. In most cases, this should not be a problem. I would certainly not eliminate a family for Marek’s that is expressed at a low rate of occurrence. Marek’s is a difficult disease to select against, but if you see from your records that a particular hen or cock’s offspring are more susceptible than the rest, you could cull those breeders and improve your results.