Infectious agents, including viruses, bacteria, and parasites, can be transmitted by human blood products. Of major importance are viruses such as human immunodeficiency virus types 1 and 2 (HIV-1/2), hepatitis B virus (HBV), hepatitis C virus (HCV), and human T-cell lymphotropic virus types I and II (HTLV-I/II). Also, other viruses such as cytomegalovirus, Epstein-Barr virus, human parvovirus B19, and hepatitis A and G viruses can be transmitted by blood products. Various methods are used to prevent transmission of blood-borne agents to recipients, such as donor selection, testing donated blood for various infectious agents, and viral inactivation of plasma derivatives. With all these precautionary measures, the estimated risk for infection by screened blood components in Europe and the United States is approximately 1 in 50,000 to 1.6 million (for HBV, HCV, and HIV-1/2) transfused blood components. In the future, the safety of blood components may be increased by testing donated blood by nucleic acid amplification techniques and by photochemical decontamination of cellular blood components.