Breast cancer is the second leading cause of cancer deaths for women in the United States and a significant cause of mortality worldwide. While there are many known risk factors for breast cancer, infectious disease has emerged as one likely contributor to carcinogenesis [1, 2]. Recent studies have implicated a number of different viral infections in breast cancer, including bovine leukemia virus [3, 4], human mammary tumor virus , human papillomavirus , Epstein–Barr virus (EBV) [7–9], and human cytomegalovirus (HCMV) [10, 11]. Although there is no clear causal role for any of these viruses, a combination of molecular and epidemiological evidence suggests an association between HCMV and breast cancer.