Several parasites can be transmitted via blood transfusion, such as Plasmodium species, Trypanosoma cruzi and Babesia microti. The risk in non-endemic areas is introduced from either travellers to, or immigrants from, high endemic areas. Either testing the blood supply or applying appropriate donor deferral addresses the risk broadly and effectively. This last strategy may not always be optimal because many healthy donors may be deferred unnecessarily, leading to donation loss, and lengthy deferrals may discourage donors to return. Optimisation of donor deferral criteria, to better reflect local donor population epidemiology, scientific knowledge and access to testing can address some of these issues.