Viruses are known to be etiologically related to several types of human cancer. In several published studies, viruses such as human mammary tumor virus, human papillomaviruses, Epstein-Barr virus, human cytomegalovirus, herpes simplex virus, and measles virus have been implicated in the pathogenesis of a subset of breast cancers (BC). However, these studies have produced conflicting results, causing considerable controversy. In this context, recent demonstration of elevated levels of mutagenic antiviral enzyme APOBEC3B in a majority of BCs is a highly significant development, as it provides a possible mechanism for development of large numbers of mutations (kataegis) that characterize many of the BCs. It has also provided further impetus for revaluating the role of viruses in the pathogenesis of BC.