I spend way too much time on the Chicken forum.
Mareks is a disease that causes the chickens to grow lesions. These can be on joints, internal organs, or about anywhere, though certain Mareks tend to grow in certain places. If the lesions grow on internal organs, it can kill them. If they grow on the neck or maybe legs, it can cripple them so they cannot eat or drink so they die if thirst or starvation. Mareks can have a devastating effect on a flock or it may not bother then that much. It depends on where the lesions grow.
The vaccine they use for Mareks in chickens is actually Turkey Mareks. That vaccine does not prevent Mareks but it prevents the lesions from growing. The chickens can still get Mareks and give Mareks to other chickens, but they will not show any symptoms. Chickens cannot give turkey Mareks to each other. I really doubt that guinea could give Mareks to your chickens but I have not researched that.
Mareks is spread by the wind. The chickens are constantly losing skin and that flakes off. Its called dander. If one chicken in your flock has Mareks they all have Mareks.
Before I got chickens I called my county extension agent who lined me up with the professor at the University of Arkansas that teaches chicken diseases. The U of Arkansas is one of the top three poultry science schools in the country, mainly because Tyson is headquartered here and heavily funds the program. That professor is also on the task force that investigates poultry disease outbreaks in this area. This area produces a lot of chickens and turkeys and they take poultry diseases seriously. Thats big money to the economy.
Anyway, I found out that there had been one reported case of Mareks in this county in the previous two years. I realize not all cases are reported, but that did not seem like a huge risk to me. I got my first ones from Cackle. They only charged ten cents per chick for the vaccination, but there was a $10 minimum. You had to get over 100 chicks before the ten cents per chick kicked in. With that $10 I elected to get 5 more chicks instead of getting them vaccinated.
I have no idea what the occurrence of Mareks is in your part of the country. If it is fairly prevalent, getting them vaccinated is probably a very good idea. If there is not much Mareks around, maybe it is less important.
For the vaccine to be effective, it has to be given a few days before they are exposed to chicken Mareks. That is not going to happen with me since I often have a broody hatch eggs. If you hatch in an incubator and keep them isolated in a brooder for a few days, it can work.
I cant tell you how important it is for you to get the chicks vaccinated. Many, many, many people do not and do OK. If you have a history of Mareks in your flock, it is a real good idea.