The presence of asymptomatic shedding of certain infectious agents poses a risk of transmission during blood transfusion. While this risk is currently well controlled for certain infectious agents (HIV, HTLV, HCV, HBV), it is not, however, documented or quantified for other pathogens responsible for infections that are either undetected or undetectable by serology at the time of donation. This risk is generally low in endemic settings, but it increases over time and across regions when clusters of cases or epidemics occur. Preventive measures can then be implemented (suspension of collection, quarantine of donations). Since these measures can have significant repercussions, particularly by limiting the supply of labile blood products to healthcare facilities, it is important that they be tailored to the risk of transmission via transfusion. Quantitative estimates of the risk of contamination in a blood donation can thus help guide these decisions. It is in this context that the French Institute for Public Health Surveillance (InVS) launched, in early 2005, a project aimed at providing a priori quantitative estimates of the risk of contamination of a blood donation by infectious agents for various scenarios in terms of incidence and spatiotemporal distribution. The objective of this article is to update the latest estimates of the residual risk of blood donation contamination by viruses routinely screened for (HIV, HTLV, HCV, HBV) and to present the work carried out by the working group "Quantitative Estimation of the Risk of Blood Donation Contamination by Infectious Agents." (R.A.)