I'm so sorry you are having to go through this. My guess, would be a wild bird brought it onto your property. Oxine, although expensive, can help, get rid of mareks on your property, although it can last for years on your land, and is now, all over...your house, your car, everywhere, you have been. There are no 100% positives with mareks. It is a herpes virus, and yes, you can carry it on your person. It is primarily spread by the chicken dander and can be spread on the wind, for miles also. This is one of the biggies, in the chicken world, and one of the hardest to eradicate from your land. As wild birds can carry it, it can be virtually anywhere, or if you have other chicken keepers in your area, it could've come on the wind also. You could've tracked it onto your property from a fair, a feedstore, or just about anywhere, since other chicken keepers, go to the same places we all do....although that risk is much lower than a wild bird bringing it, or a neighbor having it in their flock, or a friend that stopped by, that unkowningly has it in their flock. You see, vaccinating a chicken for mareks does not stop them from contracting the disease, it just stops them from developing the tumors, that kill them...they are cancerous tumors..so a vaccinated flock could be full of carriers and not even know it...there are so many ways to get this horrible disease.
Yes, mareks is a devastating disease,and there are many, right here on BYC that have it in their flocks now..there is a great link, one of many, that can get you intouch with others and help with ideas, support, and suggestions I can not. You might want to try that thread and start at the end, as those are the newest posts...it's called mareks in the flock not an emergency. There are lots of really nice people on the thread with great info and personal experience with this disease. Again, soo very sorry!!! Best wishes and hope you can find the help you are searching for with the caring people on that thread!