Marek's disease is a highly contagious viral neoplastic disease in chickens caused by a herpesvirus known as Marek's disease virus (MDV) or Gallid alphaherpesvirus 2. The other diseases are caused by different viruses: Ranikhet (Newcastle disease) by a paramyxovirus, Avian Influenza by an orthomyxovirus, and Infectious Bronchitis by a coronavirus.