The sandfly Phlebotomus papatasi transmits Leishmania major, the agent of cutaneous leishmaniasis, in desert and savannah regions of the Old World, where seasonal stress of dehydration and heat reduces the quantity of sugar in plant leaves. Without essential sugar, only a few flies that feed on leaves can survive for long enough to deposit eggs and transmit Leishmania. Accordingly, selection for hunger tolerance may also select for pathogen susceptibility in flies. Here we provide evidence of a link between these advantageous and costly properties by testing the susceptibility of flies selected by sugar deprivation and of flies from irrigated and arid habitats.