A number of viruses may be transmitted by blood transfusion, the most impor
tant of these are HIV, human T-cell lymphotrophic retrovirus (HTLV), hepati
tis B virus (HBV), and hepatitis C virus (HCV). A series of overlapping saf
ety measures are in place and are being improved and supplemented continuou
sly. As a result, the risk of transmission of these viruses in the USA has
been reduced to between one and four per million blood components transfuse
d.