 So far, we have restricted our discussion to already existing persons. Whether the benefits accrue in the present or the future, and to identified or statistical persons, the beneficiaries all currently exist. A further complication arises when we consider preventive measures that primarily benefit future persons (Parfit 1987; see also the entry on the  non-identity problem).  For instance, the eradication of polio or smallpox provides little benefit to those who have already acquired immunity (either due to vaccination or contracting the disease). The primary beneficiaries of these disease eradication programs are both vulnerable existing persons and future persons who will be born into a world without these diseases. Indeed, arguments in favor of eradication efforts often appeal to the interests of future persons and intergenerational justice (Emerson 2010; 2014). Here, familiar questions re-emerge. Should we prioritize currently existing (identified or statistical) persons over future persons? If so, there are difficult additional questions about how much more weight existing persons should receive.