Other risks such as classic or variant Creutzfeldt–Jakob disease (vCJD), an invariably fatal disease, remain worrisome as there are currently no approved tests for which to screen blood donors for this disease. Blood centers worldwide have instituted criteria to reject donors who may have been exposed to classic CJD and vCJD. Screening for transmissible diseases and deferral policies for classic CJD and vCJD designed to improve safety have unfortunately contributed to shrinking the donor pool. Blood shortages exist in the United States and worldwide. In many industrialized countries 5% or less of the eligible population are blood donors.