Yes, there are so many exceptions, but we can do our best to keep immunized (or immune) birds, but my statement ONCE YOU HAVE it on your premises--you can expect to ALWAYS HAVE IT, is a warning.. So, yes, from egg shells after chick has hatched is possible--or from a mother-setting hen who may also be vaccinated and immune, but LIVES on your premises and may have some dander in her plumage from a long ago virus--(no way to know this) could still have exposed the newly hatched chicks. BUT MAYBE NOT, worth all efforts to protect the chicks! We must try to do our best--and I do not even visit friends with unprotected chickens! Or take them out to see my birds!!!! I used to show my birds, and NEVER brought one home from a show, even if a top winner. All birds entered were sold prior to the show and new owner took them from the show--or they were shipped after the show, from the SHOW! Clothes are always different from ones worn at home, even shoes--and washed! And do not visit poultry friends on the way to or from a Show. Folks who hatch, raise and sell MANY chicks/birds would not be my choice from which to purchase birds. Hatcheries may be free of the virus or not, but for small exra cost, wil vaccinate at hatch, but they still could be exposed at the hatchery and vaccination might be too soon to protect--but worth it to try to avoid Marek's!
Never heard of it years ago. Commercial chicken farms--the thousands of birds crowded or flocked together--have brought us a number of Poultry Diseases that now affect home flocks. Coccidiosis was the main problem, in the 1930s (when I first met chicken laying flocks)--and is now easily avoided or easily treated.
Good luck to everyone who keeps, enjoys, and hatches chickens/chicks of any breed or type!