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Breast Cancer (BC) is one of the most prevalent of all cancers worldwide. It is a well-established disease with intrinsic and extrinsic risk factors. Over the past decades, scientists postulated a role for infectious agents and the resident microbiota as extrinsic risk factors for several types of cancers. Viruses may exert effects on the early stage of oncogenesis during the initiation or late stage through the regulation of cell proliferation and apoptosis. Bacteria within the host may interact with host cells, such as the epithelium and immune cells to affect development or progression of BC. For example, microbes may impact the host response, from somatic or immune cells, causing changes in inflammatory pathways and the tissue microenvironment, which may influence cancer development. Microbial communities composed of eukaryotic species, bacteria, fungi and viruses inhabit the human body and may contribute to cancer pathogenesis or prevention. This chapter describes studies related to the associations of BC and microorganisms present in humans discovered over the last decades.