For the past 30 years, the poultry industry has relied on a series of avirulent or attenuated live virus vaccines to provide protection to young chickens against natural challenge with field strains of Marek’s disease (MD). This strategy has been unusually effective. In the United States, losses from the condemnation at slaughter of young broiler chickens with visible lesions of MD have decreased from 1.5% in 1970 to 0.0121% in 1999, a reduction of over 99% (Fig. 1). Even more dramatic reductions are evident in Georgia and Delaware, states with intensive broiler production. Similar benefits have been realized in commercial layer and breeder flocks. Vaccination for the control of MD in the field has been one of the great successes in veterinary medicine.KeywordsProtective EfficacyInfectious Bursal Disease VirusMaternal AntibodyFeather FollicleAvian PatholThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.