The transfusion of blood and blood components is associated with a very low but ever-present risk of infection. It is estimated that 1 in every 2000 units of blood may carry an infectious agent and that about 4 in 10 000 recipients develop a chronic disease or die as a result of receiving contaminated blood. A wide variety of viral, bacterial, and parasitic agents have been associated with blood transfusion (Table 104.1). Concerns have also been raised about the potential for transmission of Creutzfeldt–Jakob disease (CJD) and its new variant (nv-CJD) through blood products. However, no human episodes of CJD or nv-CJD have been causally liked to blood transfusion, and case–control studies have not found blood transfusion to be a risk factor for CJD. The risk of viral transmission has been markedly reduced with improved screening, particularly using nucleic acid testing (NAT). The risk is now estimated to be 1 in 2 million units for human immunodeficiency virus (HIV) or hepatitis C virus (HCV) and approximately 1 in 200 000 units for hepatitis B virus (HBV). Because the risk of viral or parasitic infection is very low and blood is screened for HCV, HBV, HIV, and human T-cell lymphoma/leukemia virus (HTLV) 1, the remainder of this chapter focuses on bacterial complications of blood transfusion, which can be diagnosed and treated.