Cancer remains one of the major challenges facing modern Human Medicine. It is also a major challenge in Veterinary Medicine, particularly in poultry where it is a major risk to the 55,000 million chickens produced commercially every year. Unlike in humans, where more than 80% of the cancers are non-infectious, the vast majority of avian neoplastic diseases are caused by viruses. Two major groups of avian oncogenic viruses that threaten poultry health are the retroviruses and herpesviruses. In addition to their role in inducing diseases in chickens, avian oncogenic viruses have been instrumental in laying the foundations for much of the basic understanding on the molecular mechanisms of cancer. Thus the oncologist's debts to avian tumour viruses are enormous. For example, a number of major discoveries including those by Peyton Rous (Rous, 1911), Howard Temin (Temin, 1976), David Baltimore (Baltimore, 2006), Michael Bishop (Bishop, 1990) and Harold Varmus (Varmus, 1990), all of whom were subsequently awarded Nobel prizes, have come from studies avian viruses.
