Understanding parasite virulence is a challenge for evolutionary biology. Intuitively, parasites that rely on their hosts for their fitness should be benign, yet many parasites cause harm (virulence). Two main theories exist to explain parasite virulence. First, it is believed that virulence is an unavoidable consequence of a parasite maximising its fitness: in order to obtain transmission to the next host, it needs to exploit its current host, at the same time causing damage. Second, within-host competition between parasite strains infecting the same host could select for higher levels of virulence, because selfish competition will favour those strains that exploit their hosts more rapidly than competitors. Although many theoreticians have addressed these theories, there is hardly any experimental evidence to confirm them, severely limiting a proper understanding of the ecological processes (and human interventions) that shape parasite virulence evolution. I propose to study these two theories experimentally , using Ophryocystis parasites in monarch butterflies (Emory University) and malaria parasites in mosquitoes (University of Edinburgh). Carrying out laboratory and field studies on monarch parasites at Emory University, I will obtain expertise in studying naturally occurring insect-parasite systems, which I will transfer to studying malaria parasites in mosquitoes upon my return to Edinburgh. Carrying out laboratory studies on mosquitoes first, I have a long-term vision of taking these studies further to study malaria virulence evolution in the field; with human interventions already well under way, experimental studies on parasite virulence are long overdue. Combining the knowledge of two different systems and the infrastructure at two different institutes will not only enhance our understanding of parasite virulence, but also greatly contribute to my scientific maturation and my career in evolutionary biology of disease, which I wish to realise in Europe.