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Viral complications of modern blood transfusion are negligible, but transfusion can never be called totally safe. The most important risks are transmission of hepatitis and human immunodeficiency (HIV) viruses. Systematic screening of blood donors for HBsAg greatly reduced the incidence of post-transfusion hepatitis B, but virus transmission is still possible in the convalescence period when HBsAg test has become negative. To avoid this possibility, some countries have started additional screening for anti-HBc. The decision depends on the prevalence of hepatitis B, and no universal recommendation can be given. The great majority of non-A, non-B post-transfusion hepatitis cases seem to be caused by hepatitis C virus (HCV). The second generation tests for anti-HCV are very useful for screening blood donors, and surrogate testing (e.g. ALT and anti-HBc) is not needed anymore. Careful donor selection and other preventive strategies including systematic laboratory testing for anti-HI V have greatly reduced the risk of HIV transmission, but in the seronegative phase of the disease transmission of infection is still possible in donated blood. Antigen screening in donors does not solve this problem. Cellular products may transmit HTLV and CMV. In the latter case, depletion of blood products of leukocytes is as good a strategy as using seronegative donors. The prevention of transmission of all viruses includes, in addition to donor selection and laboratory screening, viral inactivation of products. This has been achieved with most plasma derivatives, and research is active with cellular products. Different preventive strategies should be used simultaneously to minimize the risk as much as possible.