It's Marek's, I think. Right, though. Worst website design in the world here but,

"The dander from an infected bird only has to be inhaled by another bird for the strain to be transmitted. In fact, since infected birds are constantly shedding the virus, a chicken coop that contained the virus can be contaminated long after the birds are gone. "

https://www.freedomrangerhatchery.com/blog/what-is-mareks-disease-and-how-to-not-lose-40-of-your-flock/