Marek's disease (MD) is a common disease of chickens worldwide, including Australia, caused by the Marek's disease virus (MDV), a cell-associated alphaherpesvirus. The virus is lymphotrophic, but also infects cells of the feather follicle epithelium in which it replicates rapidly and is shed in feather dander, being transmitted laterally via inhalation of this material. The disease as first described by Josef Marek in 1907 was characterised by paralysis and lymphocytic infiltration of peripheral nerves in older chickens. This form of the disease, known as "classical" MD was superseded in the 1950s and 60s by "acute" MD characterised primarily by lymphomas in multiple organs in younger chickens. This development, associated with the intensification of the poultry industry, was a major threat to the industry until the concurrent discovery of the causative agent and development of live vaccines in the UK and USA in 1969. Since that time, there has been an ongoing evolution of virulence of MDV in countries such as the USA, associated with sequential vaccine failure and greater virulence in unvaccinated chickens with or without maternal antibody directed against MDV. Marek's disease vaccines are "imperfect" in that they protect against MD but not infection and shedding of virus, and as such are implicated in the evolution of virulence of the virus. Recent modelling work, based on our Australian research has shown that both vaccination and the reduced host lifespan that has occurred with the intensification of the chicken meat industry are implicated as causes of the observed increase in virulence.