A recent investigation has identified transplanted organs as the source of WNV infection in four recipients or organs from a single donor. How the organ donor became infected is unknown. The organ donor might have become infected from a mosquito bite or possible acquired the infection through transfusion; an investigation of the numerous transfusions received by the organ donor is ongoing. Since the report of these cases, the Centers for Disease Control and Prevention (CDC) has been informed of other patients who developed WNV infection within several weeks of receiving blood products or organs. Investigations are ongoing to determine whether WNV was transmitted by transfusion or transplantation in any of these cases.