For me it started with one lame one and like everyone you automatically think injury but then a second and third with the e type of lameness was too much of a coincidence. I had had deaths in one or two older birds prior to that, some quite sudden and others lingering, but then the 6month old youngsters going lame was the moment I became suspicious and then I did a post mortem exam on the first one that I decided to euthanize and found large tumours on her leg and abdomen.
I did try turmeric and black pepper mixed into scrambled egg as that is supposed to reduce tumours but after a week or so they got sick of it and it just stressed me and them trying to get them to eat it. Now I feed the sick ones whatever they will eat. I have found that having the companionship of another hen in sick bay stimulates them to compete for food and keeps them from being depressed and getting them out on grass in sunshine (in a cage) but with the rest of the flock foraging around them makes a huge difference. Sadly here in the UK sunny days are limited, especially at this time of year and it's too cold and wet or them to go out on the grass, so all I can do is make them as comfortable and as happy as possible and hope that they will survive until the spring. I had to cull a cream legbar cock last week that had his first attack a year ago, when he was suddenly badly lame for a few days and then completely recovered over a couple of weeks and has shown no sign of any more problems until 2 weeks ago when I found him on his side and unable to get up. His leg had gone again and his neck was also starting to twist. He was still eating and drinking though, so I kept him warm and comfortable, but then his system started to shut down and he wouldn't eat and drink anymore and I knew it was time to call it a day.
On the other hand, I have a young pullet that started with paralysis of her wing 6 weeks ago. It was so bad she was tripping over it. I trimmed the feathers to help a bit and put her in the infirmary with her brother who has persistent pasty butt which I suspect may also be Marek's related. They have both improved to the point that I let them out to free range with the flock through the day and they stay in the infirmary overnight, where I can monitor poop and I know they have good access to food and water. Both are under size but they have good quality life. My infirmary/brooder is in the hen house so that they can remain part of the flock.... My other birds have all been exposed to the virus so I don't isolate Marek's sick birds.
I am lucky that the strain I have is not as virulent as some members of this forum experience. I've had 28 broody reared chicks this past summer all reared within the flock and only these 2 have shown possible signs of Marek's. There are 5 pullets just coming into lay.... got my first egg from one yesterday, so this is a critical time... Marek's strikes at times of stress and the confusion of hormones and egg laying can trigger an outbreak..... as can over enthusiastic young cockerels trying to mate them when they are not ready.
There are several very good threads on Marek's on this forum and I know there are people trying human anti virus drugs on their sick birds and others trying St John's Wort. The difficulty is that some birds experience quite dramatic recoveries, with or without medication so it's very difficult to assess efficacy and of course others decline and die quite quickly regardless. I now just give supportive care.
Whatever it is that is ailing your flock I wish you luck in dealing with it and if I can give you any further help just ask.
PS. If it is Marek's, keeping your flock as stress free as possible helps prevent outbreaks. Therefore avoid overcrowding, keep adolescent cockerels separate from the females (a bachelor pad for the boys is good until they get over the raging hormones phase) and ensure plenty of feeding and water stations so that all have access to food without risk of bullying.