Orphan drug policy often gives ‘special treatment’ to rare diseases, by giving additional priority or making exceptions to specific drugs, based on the rarity of the conditions they aim to treat. This essay argues that the goal of orphan drug policy should be to make prevalence irrelevant to funding decisions. It aims to demonstrate that it is severity, not prevalence, which drives our judgments that important claims are being overlooked when treatments for severe rare diseases are not funded. It shows (...) that prioritising severity avoids problems caused by prioritising rarity, and that it is compatible with a range of normative frameworks. The implications of a severity-based view for drug development are then derived. The severity-based view also accounts for what is wrong with how the current system of drug development unfairly neglects common diseases that burden the developing world. Lastly, the implications of a severity-based view for current orphan drug policies are discussed. There are no data in this work. (shrink)