Marek's manifests in multiple ways. There is the occular version, which shows up as cloudy eyes and misshapen pupils; the neurological version, evidenced by birds who suddenly can't stand and walk because one leg won't work, and ultimately the legs stiffen straight out; the internal ,called visceral, where the bird develops tumors on internal organs; and there is also one called cutaneous Marek's, where the birds develop tumors on the feather follicles. More than one type can be present, and sometimes a bird starts off having one and then develops another type. The neurological version is considered "classic" Marek's, and it is unmistakeable. The occular version is also very obvious, the birds have trouble finding food and water, and will peck near it before they hit it. When you pick them up to look at them you immediately see the cloudy eyes and elongated pupils The visceral is harder to spot, because they are eating and can see and walk, but one day you pick them up and they are bones and feathers, or they just die.
Some birds have natural immunity/resistance to the virus (sort of like some people will get chicken pox and some won't because they are born with natural immunity), and some are exposed to the turkey version, which is mild to the extent you don't even know they are exposed and their immune systems are working to fight something. The accepted theory is that infection with the turkey virus "teaches" the immune system that this virus is something to fight and kicks it into gear without making them sick in an obvious way, so that when they are exposed to the chicken virus their immune systems are already in full fight mode.
The reason the vaccine is a problem is that it is a live virus vaccine, and once the birds are vaccinated they shed the virus the rest of their lives in their feather dander. When I put unvaccinated chicks in the same vicinity as vaccinated chicks, I exposed them, and those that were susceptible became ill and died. From everything I've read, once the birds are older than 6 months they usually won't get sick or die from it, their immune systems have matured enough to fight it off.
It is possible fresh air and sunshine reduces risk, it's certainly good for the overall health of the birds, but if it can be spread by dander of wild birds, I don't see any way to completely avoid it. Even if you kept your birds in a bubble, you could walk on the dander, brush against a tree where dander has landed, pick it up at the feed store, WalMart, etc. - when they say it's everywhere, that is a very literal statement. It is not more common in one area than another, so it is unlike Fowl Pox, which likes warm and moist climates. Even coccidiosis is more prevalent in warm and moist areas - those in the Southeast and in coastal areas almost all use medicated chick starter because it is so common, where I have not used medicated feed at all this year and have not (knock wood) had any problems with coccidiosis. Marek's is a whole 'nother ball game.
