This guidance document has been produced by WHO to assist blood services in the development of national plans to respond to any emerging infectious threats to the sufficiency or safety of the blood supply, whether from an existing infectious agent that is changing in incidence and spread, or from a newly identified infectious agent. It is intended that this document be followed to guide the national blood service through the process of planning how to respond in a timely, controlled and appropriate way to any specific infectious threat that may subsequently emerge. It is acknowledged that it is not only the blood supply that may be affected by such emerging infectious threats; in those countries undertaking transplantation, the supply of cell, tissues and organs may also be threatened. Increasingly, blood services are taking overall national responsibility for transplantation in their capacity as the organization responsible for the collection, processing, storage and supply of cells, tissues and organs. This approach is both sensible and appropriate, as the overall donor selection and screening processes are the same or very similar. This guidance document can therefore also be used to assist those bodies responsible for the provision of cells, tissues and organs to prepare for an emerging infectious threat.