Following the health crises of the 1990s (contaminated blood, mad cow disease, asbestos, . .) and more recent ones caused by the 2003 heat wave or emerging infectious pathogens (SARS, West Nile, Chikungunya, H5N1, H1N1. . .), a comprehensive public health surveillance system has gradually been established in France. After a brief historical overview of the public health alert system, this article will outline its current organization in France and present two examples of recent health alerts (chikungunya in Réunion in 2005–2006 and the hepatitis A outbreak in the Côtes-d’Armor in August 2007) that required the implementation of preventive measures regarding the selection of blood donors. These two examples, among others, have shown that the role of alerts within the French health surveillance system is crucial to ensuring that alerts are issued as early as possible, in close proximity to the event in question. Thus, it serves as an early decision-making tool, particularly when it comes to implementing measures to prevent the transmission of a transfusion-transmissible agent. (R.A.)