We identified a novel class of small circular single-stranded DNA agents as human pathogens. They are related (but not identical) to specific bacterial plasmids. Most of them were isolated, sequenced and characterized from Eurasian dairy cattle sera or milk products and designated as Bovine Milk and Meat Factors – BMMF. Some have been directly isolated from human colon and breast cancers and a lesion from a multiple sclerosis brain autopsy. Human infections occur by nutrition early in life, resulting in persistent chronic infections for lifetime. Foci of infected cells can be detected by specific monoclonal antibodies, directed against a replication-linked protein (rep), commonly in close proximity to actively replicating cells of Lieberkühn’s crypts in the colon. The stained cells did contain BMMF genomes. We developed a model of indirect carcinogenesis by BMMF for colon and for breast cancer development.