Marek's disease virus (MDV) is a highly contagious virus that primarily affects chickens. It belongs to the Herpesviridae family and can cause tumors and other serious health issues in infected birds. The virus spreads through the air and can remain dormant in the environment, making it difficult to control. Infected chickens may show symptoms such as paralysis, weight loss, and changes in feathering. Vaccination is a common method to protect poultry from Marek's disease, helping to reduce the impact of the virus in commercial and backyard flocks.