Marek’s disease (MD) is a lymphoproliferative disease of chickens. Various symptoms characterize MD in chickens, including neurological signs, metabolic disease, immune suppression, and lymphoma development. MD is caused by gallid alphaherpesvirus 2 (Mardivirus gallidalpha2), or Marek’s disease virus (MDV). The virus is endemic in most of the world and causes economic losses estimated to be $1–2 billion USD annually. The virus is highly infectious to numerous avian species but is predominantly a significant threat to chickens. Infection of B and T cells are the primary cell types infected, and latency is established in T cells where the virus is maintained for the host’s life. Dissemination occurs via the shedding of infectious virus and infected cells from the epithelial skin cells surrounding feather follicles where the virus is present in the dust and dander. The stability of infectious MDV in poultry houses (weeks to months) makes it difficult to eradicate from houses. New chicks are immediately exposed to infectious virus and are infected through inhalation of infectious dust and dander. MD is controlled by vaccination, but vaccination does not prevent infection that has resulted in increased virulence over the decades. MDV mostly affects the poultry industry, but backyard flocks are also affected.