One living virus or even a hand full may or may not spread Mareks. The dandruff produced from new feather growth definitely will because it is associated with pinfeathers and contains a lot of dried chicken blood and this dandruff is loaded with Mareks viruses and will pass Mareks to nearby chickens as a dust. Mere exposure is not the problem however the way that exposure occurs can be devastating. Close range isolation is ineffective at stopping Mareks because it is spread on the breezes or zephyrs. That is why I would never bring a Mareks carrier or "gander eyed" bird onto my property, it is just inviting future problems that I don't need. So is treating or keeping and treating a Mareks infected bird because you are only setting up a Mareks nursery with each and every bird whose symptoms you LOL succeed(?) in curing.