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There are a number of neoplasias for which a herpesvirus is an essential part of the aetiology. Of these, Marek's disease is the most common and provides excellent opportunities for the study of a herpesvirus-induced tumour both experimentally and under natural conditions in the field. Marek's disease is caused by an alpha herpesvirus; it differs from the other oncogenic herpesviruses which are gamma herpesviruses. It is a ubiquitous virus in poultry populations of the world and is highly cell-associated and contagious, yet only a proportion of infected fowl develop tumours. Evidence is presented to suggest that at least one of the reasons for a wide variation in the incidence of the disease is a temporal interplay between virulent viruses and viruses of low or no virulence. The viral genes associated with the oncogenicity of Marek's disease virus (MDV) are discussed and it is concluded that it is likely that several genes are involved. Finally, a brief history of vaccination to control Marek's disease is given and mode of action discussed. It is concluded that the mechanism of protection is mainly through an antiviral cell mediated immune response, resulting in a lowered challenge virus burden. Marek's disease viruses over the past 40 years have been evolving greater oncogenicity, some of which are not adequately controlled by the vaccines that are currently available. It is suggested that for MDV to produce tumours, there is a need for the cytolytic infection phase and that infection must be with an MDV which possesses a functional gC, ICP4 for maintaining latency which allows the expression of at least the 1.8 kb family, pp38, meq, and possibly pp14 genes, for maintaining the tumour state and possibly initiating this state. Intervention in this process reduces the chance of tumour formation and incidence in a population which can occur through natural or man-mediated infection with non-pathogenic MDVs.

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1. Immunisation was first practised as early as the 10th century when small doses of smallpox material administered by unusual routes were used to immunise against smallpox. The procedure was introduced into England in the early part of the 18th century. 2. The next major development was the use by Jenner of cowpox to vaccinate against smallpox in the late 18th century. 3. Some eighty years later came the classic studies of Pasteur developing vaccines for fowl cholera, anthrax and rabies. 4. The studies of Jenner and Pasteur established the major principles of vaccination which are in use to this day. 5. The major viral diseases of the domestic fowl were recognised during the 1920s and 1930s and in most cases vaccines were developed within 5 years of the discovery of the viral nature of the cause of each disease. 6. The desirable properties of poultry vaccines required by the user and producer are not completely fulfilled by currently available vaccines. 7. There is a need to use the opportunities provided by modern biotechnology and immunology to search for and develop vaccines that better fulfil the desirable properties of poultry vaccines. 8. There are a number of strategies available for the development of novel vaccines, some of which are appropriate for the needs of poultry vaccines.