Preventing the risk of human-to-human transmission of emerging or re-emerging pathogens in products of human origin is a concern for health authorities. Traditionally, these infectious agents—which are not screened for in blood transfusions due to certain characteristics (low infection incidence, short and transient presence in the blood, etc.)—can, under specific epidemiological circumstances (clustered cases, epidemics, etc.), a public health problem that warrants their surveillance and the potential use of preventive measures. Each year, the emergence or re-emergence of infections caused by arboviruses (such as the West Nile virus) in many countries requires special attention. Alerts transmitted by surveillance systems are analyzed by a decision-support unit that selects the most appropriate preventive measures. Prevention of prion infections is achieved by excluding donors with a family history of neurodegenerative disease or who have received treatment with extracted growth hormones or during a transplant. Screening was strengthened in 1997 with the exclusion of donors who had previously received a blood transfusion, and in 2001 with the exclusion of donors who had spent a cumulative total of more than one year in the United Kingdom between 1980 and 1996, a period of risk due to the bovine spongiform encephalopathy (BSE) epidemic in that country. Prevention of parasitic infections (malaria and Chagas disease) is achieved through the temporary deferral of blood donors exposed to the risk of infection (four months after returning from a risk area), supplemented by serological screening. (R.A.)