Mareks is a virus common in chickens. It can cause tumors on the nerves, causing paralysis of one or both legs or wings, or a twisted neck. Tumors may develop on eye nerves causing blindness, on the skin, and on internal organs. It also causes a lack of immunity to many diseases. There is a vaccine available for day old chicks, which need to have 2 weeks to develop immunity. The virus is widespread in the world, is spread through direct contact and from dust or dander, some chickens may have natural immunity, and the virus can be found in the environment for several years. You can Google “Mareks disease” for more info.