Everything is the same with your flock as it was yesterday. The only difference is you have this new piece of information to guide how you care for your flock and how you practice bio-security to prevent this disease from escaping your premises and other diseases from coming in to lower the resistance of your chickens.
The first thing to understand is that only some of your chickens will become symptomatic. This is not a death sentence for your flock of fifty chickens. With good care, maximizing the health of your flock, many, if not most, will develop resistance to the disease, and while all carry the virus, not all will become sick and they will lead full, healthy lives.
The main precaution will be to contain the virus by taking care to have shoes you only wear with your flock that will not be worn off your property. You will not rehome or sell any chickens from your flock. You may still import new chicks and chickens, and there is an easily obtainable vaccine you can give the new chicks and chickens yourself, quarantine them until they develop resistance, then they can become full flock members.
Other than that, deal with each chicken that becomes symptomatic, explore various treatments that can prolong the life of a symptomatic chicken, and euthanize when quality of life is at an end, as you would any flock member that has become sick.
Recently, I had three pullets become symptomatic in my flock that carries the lymphoid virus. I explored many treatments that appeared to stave off symptoms, all documented in this thread.
https://www.backyardchickens.com/th...update-now-another-pullet-going-lame.1432738/
One of the things you can try is an antiviral prescription med that treats the herpes virus, of which Marek's is one. It's call acyclovir, and it's used to treat herpes mouth sores and shingles. It may help retard the progress of some forms of Marek's. Some folks just mention to their primary doctor that they have lip sores and need the prescription. There is no cure for Marek's, but acyclovir, vitamins B and E and good nutrition and keeping premises as bacteria free as possible will all contribute to the well being of a flock carrying an avian virus.
My flock is testimonial that chickens can live long, healthy lives in spite of carrying an avian virus. Currently, my oldest hen is still going strong at thirteen years plus, in spite of having a growing tumor over her right eye for half her life.