Do you have wild turkeys where you live? Or can you get some turkey litter? Turkeys carry a less virulent strain of Marek's disease, so provide natural immunity. If you've had your birds immunized, you need to be aware that the immunization does not confer immunity. All it does is keep the disease from moving to the "tumor" stage. Your birds can still be infected, but will not present with noteable signs. So, they can still pass the disease on to other birds. It can be brought in by just about any bird traveling through your property, if I have my facts straight.