The occurrence of asymptomatic penetration of certain infectious agents in blood presents a risk of transmission of one of these agents during blood transfusion. Although well controlled for some infectious agents (HIV, HTLV, HCV, HBV), this risk is nevertheless neither documented nor quantified for other pathogens that are responsible for serologically unscreened or undetectable infections at the time of blood donation. This risk is generally low in endemic situations, although it increases for particular time periods and locations when clustered cases or outbreaks occur. Prevention measures may then be implemented (interruption of blood collection, quarantined donations, etc.). These measures can have an important impact, particularly by limiting the supply of blood products to health care facilities. It is therefore important for these measures to be adapted to the risk of transmission through blood transfusion. Quantitative risk estimates of blood donation contamination can therefore contribute to guiding those measures. In this context, in early 2005, the French Public Health Institute (InVS) started a project with the aim of obtaining a priori quantitative risk estimates of contamination of a blood donation by infectious agents for various scenarios in terms of incidence and time-space distribution. The objective of this article is to update the last estimates of residual risks of the major transfusion-transmitted viral infections (HIV, HTLV, HCV and HBV) and to present the work realized by the working group << Quantitative estimate of the risk of blood donation contamination by infectious agents>>.