Provided herein, inter alia, are methods for predicting the occurrence of metastasis in an individual affected with breast cancer as well as methods for determining the risk of metastasis in individuals diagnosed with breast tumors by assaying for the presence of viral interleukin-10 (cmvIL-10) in individuals diagnosed with or suspected of having breast cancer that are seronegative for human cytomegalovirus (HCMV).
