As new agents of infectious disease continue to emerge and old antagonists reemerge, it is clear that the war on infectious disease is far from over. Indeed, the appearance of SARS during the past year is the latest example of the continuing challenges posed by agents of infectious disease. Emerging agents are threats not only to the general population, but also to recipients of blood transfusions. Today a variety of emerging agents are of concern to transfusion safety including Trypanosoma cruzi, West Nile virus, and Babesia microti to name but a few. These and other emerging agents have arisen or have been introduced partly through changes in donor demographics, international travel, microbial adaptations, land use, and human behaviour. Unfortunately, for many of these agents there is an absence of viable tests or other sound interventions to prevent their transmission at this time.