Marek's disease is a Herpes virus that is ubiquitous in the environment. If you've had Marek's in your flock, the virus can be present for 8-12 months in the environment after the last infected bird has died and unless you wait 8-12 months to replace your flock, you will start the cycle all over again when you get new birds.
Vaccination is done in 18-19 day eggs or as one day old chicks. These chicks do shed virus from the vaccine, but it is NOT infective to other chicks/birds. Marek's is shed in dust and fluff from infected birds (if your bird if outside, it's probably infected) and these materials should be kept away from vaccinated chicks for 2-3 WEEKS. If vaccinated chicks come in contact with infected materials (clothes, equipment, housing etc), they can contract the disease since their immune system may not have adequately responded to the vaccine in that time period. Chicks in poor health (parasites, poor diet etc) may not adequately respond to the vaccine and may not be protected. Vaccinated chicks who are subsequently exposed to Marek's in the environment WILL shed virus, but (hopefully) will not succumb to it.
Unvaccinated chicks are exposed with days/hours of introduction to a contaminated area. Some will appear healthy, but shed virus (often for life), some will succumb in a few months, some may go a few years. There is no treatment and the vaccine is not effective once the bird has been exposed.
The adult vaccine is available in some countries, but is not considered useful since most birds are exposed within days of entering a contaminated environment.
Like all viruses (think influenza), there are different strains. Current vaccines are HVT (Herpesvirus of Turkeys) and/or Rispens. The Rispens vaccine has proven effective against a more virulent strain of Marek's that emerged in the 1990s and caused significant mortality even in vaccinated poultry.
