As noted in Chapter 1, the possibility that infectious agents are a significant cause of neoplasia was quite popular in the latter part of the nineteenth century. However, the popularity of this concept decreased during the twentieth century until about four decades ago, when several of the oncogenic viruses became known and subsequently were characterized both biologically and in a molecular sense. It is difficult to ascertain the importance of infectious agents as causes of neoplasia in lower forms of animals in their natural habitat, but it is apparent, as can be seen from many of the discussions in this chapter, that infectious agents can be a significant causative factor for neoplasia, either induced or spontaneous, in domestic and laboratory animals. As dis- cussed in Chapter 12, infectious agents as causative of human cancer may represent as much as
15% of the total cancer problem. While this may seem alarming, it also presents the possibility that various preventive measures such as vaccination, better personal hygiene, and public health measures can potentially control if not eliminate a number of infectious causes of human neoplasia.