Marek’s disease is one of the most widespread poultry diseases in the world. The first report of the disease was in 1907 by József Marek, for whom the disease is named. Since the initial discovery, the disease has been found worldwide and is a major concern to those associated with chicken production. It is a highly contagious viral disease caused by a herpes virus called Alphaherpesvirinae. The virus has the ability to invade the body and survive without being destroyed by the immune system. The virus infects certain white blood cells, causing some of these cells to become cancerous. These cells can then infiltrate internal organs (kidneys, liver, gonads, and proventriculus), peripheral nerves, skin, and muscles. Marek’s disease can occur in chickens 3 to 4 weeks of age but is most commonly seen in birds between 12 and 30 weeks of age. Female birds are more often affected than are males.