Marek's disease (MD) is a highly contagious lymphoproliferative disease of poultry caused by the oncogenic herpesvirus designated Marek's disease virus (MDV). MD has a worldwide distribution and is thought to cause an annual loss over US$ one billion to the poultry industry. Originally described as a paralytic disease, today MD is mostly manifested as an acute disease with tumors in multiple visceral organs. MD is controlled essentially by the widespread use of live vaccines administered either in ovo into 18-day-old embryos or into chicks immediately after they hatch. In spite of the success of the vaccines in reducing the losses from the disease in the last 30 years, MDV strains have shown continuous evolution in virulence acquiring the ability to overcome the immune responses induced by the vaccines. During this period, different generations of MD vaccines have been introduced to protect birds from the increasingly virulent MDV strains. However, the virus will be countered each new vaccine strategy with ever more virulent strains. In spite of this concern, currently field problem from MD is likely to be controled by strategy of using bivalent vaccine. But, potential risk factors for outbreak of MD are still remained in this condition. The major factors can be thought that improper handling and incorrect administration of the vaccine, infection prior to establishment of immunity, suppression of immune system by environmental stress and outbreaks of more virulent MDV strain by using vaccine and genetic resistance of host.

Marek's disease virus (MDV) is a highly cell-associated, lymphotropic α-herpesvirus that causes paralysis and neoplastic disease in chickens. The disease has been controlled by vaccination which was provided the first evidence for a malignant cancer being controlled by an antiviral vaccine. Marek's disease pathogenesis is complex, involving cytolytic and latent infection of lymphoid cells and oncogenic transformation of CD4 + T cells in susceptible chickens. MDV targets a number of different cell types during its life cycle. Lymphocytes play an essential role, although within them virus production is restricted and only virion are produced. Innate and adaptive immune responses develop in response to infection, but infection of lymphocytes results in immunosuppressive effects. Hence in MDV-infected birds, MDV makes its host more vulnerable to tumour development as well as to other pathogens. All chickens are susceptible to MDV infection, and vaccination is essential to protect the susceptible host from developing clinical disease. Nevertheless, MDV infects and replicates in vaccinated chickens, with the challenge virus being shed from the feather-follicle epithelium. The outcome of infection with MDV depends on a complex interplay of factors involving the MDV pathotype and the host genotype. Host factors that influence the course of MD are predominantly the responses of the innate and adaptive immune systems, and these are modulated by: age at infection and maturity of the immune system; vaccination status; the sex of the host; and various physiological factors.