So sorry to hear they found tumours and Marek's is the likely cause. I have had the disease in my flock for 4 years now and it has been a steep learning curve. I would be lying if I said there has not been heart ache but it does settle down over time and there will be birds that are resistant to it and some that recover from outbreaks and this initial outbreak is probably the worst period if that is any consolation. Once you know what it is and learn how best to manage it, it becomes just another challenge to face along with predator attack and pests and parasites and all the other ailments that chickens are subject to. It may seem really scary when you first start researching the disease and I certainly went through a panic phase when I got the diagnosis but chickens can and do survive it and you have to rejoice in those rather than dwell on the ones you lose. You may want to purchase some Virkon S disinfectant which is effective in killing the virus. You can buy small quantities, so don't be put off if you initially just find expensive bulk purchase items. I just bought some sachets, that you sprinkle into a bucket of water for £1.99 each. The chicken coop is the most likely place for infected material to remain.... it is shed in dander dust and can stay viable in the coop for months if not a year, so a good clean out and disinfect after you have had an outbreak is probably beneficial. The infected dust is inhaled by other birds to transmit the virus to them and obviously the coop, where birds are flapping their wings to get up and down from roost bars etc makes it very easy for that infected material to become airborne and inhaled, so a good clean out and spray down with Virkon S will reduce the reservoir of infected material in your flock's environment significantly. Birds with Marek's generally only shed the virus when they are symptomatic, so it's not like all the birds are shedding the virus all the time and you need to constantly disinfect. The virus can be carried on your clothes skin, hair, shoes and even the wind, so I am not saying this will prevent other birds from getting it and some of your remaining flock may well already have been infected, but to me it makes sense to keep the infected material in their environment as low as possible. The disease has dormant phases and outbreaks and there is a minimum 3 week dormant phase between infection and a bird becoming symptomatic, but it can be several months or even years. Outbreaks are usually triggered by stress, so keeping the flock as happy and healthy as possible is important. Things like surge of hormones at point of lay or moult or severe weather are not something you have control over but changes to their management and environment should be minimized. Adolescent cockerels running amok in the flock are one of the commonest triggers. Marek's suppresses the immune system, so giving the birds a vitamin boost with a proprietary poultry supplement at times of stress can be helpful. Anyway, I know you will have lots of reading to do, but try not to feel overwhelmed. There is life after a Marek's diagnosis, but it takes a while to start seeing the light. If you need any advice or support, feel free to ask. There are many of us here at BYC who have this disease in our flocks and are learning to deal with it.