The reconstitution of blood and its components is hampered by factors of compatibility, availability, and the risk of transmission of infectious diseases. Protozoal agents such as plasmodium malariae and trypanosoma cruzi are only regionally relevant. Bacterial transmissions are easy to prevent and treat. Antibody, antigen, and nucleic acid screening have been implemented to prevent transmission of blood-borne viruses. Transfusion-relevant viruses include hepatitis B and C virus (HBV and HCV), human immunodeficiency virus (HIV), human T leukemia virus (HTLV-I), and in certain circumstances, parvovirus B19, hepatitis A virus (HAV), and cytomegalovirus (CMV). Of great concern is the possible transmission of prion protein causing transmissible spongiform encephalopathy. Of future interest will be whether other viruses such as Nipah and Hendra virus are blood-borne and whether viruses such as TT, SEN, and GBV-C are involved in diseases or their progression, while not causing hepatitis.