An overview of recent studies of antimicrobial factors and microbial contaminants found in human milk is presented. The incidence of gastrointestinal and respiratory infections in infants receiving human milk continues to be lower than in those not breast-fed due to the presence of specific antibody and possibly anti-adhesion factors in the milk. Whether the many other antimicrobial factors, which have been shown to be active in vitro or in animal model systems, have any influence on infant infections is still not clear. Microbial contaminants in human milk are rare, as are associated infant infections from the milk. However, some contaminants such as cytomegalovirus are commonly transferred to infants from the milk of seropositive mothers, fortunately without any adverse effects in the infants. Human T-lymphotropic virus type 1 is transferred via human milk in endemic areas, human milk being the main source of mother-to-infant transmission. While some reports suggest human immunodeficiency virus type 1 transfer may occur through human milk, this is not the predominant mode of transmission to infants.