Paget's bone disease is a chronic and usually benign bone disorder mainly found in people over fifty. The condition is characterized by excessive bone remodelling related to bone cell overactivity leading to abnormal bone structure. The viral origin of the disease, first proposed on the basis of ultrastructural observations, is now supported by immunocytological and molecular hybridization studies. Genetic and environmental elements may be involved, favouring and modulating the expressivity of this slow virus infection.