Thought experiment time! Let’s say you could somehow (1) find/identify/locate and (2) eliminate every parasite on the planet. In this case we’ll lump pathogens and viruses and parasitoids and maybe even micropredators/vectors into the category of “parasites.” If you waved your magic wand and eliminated parasites from the planet, what effects would that elimination have on individuals, populations, communities, and ecosystems? I think that’s a great disease ecology prelim question, and you should all start using it immediately.
If you’re interested in the answers that other people have proposed to answer that question, I have some links for you! You can check out Wood and Johnson’s recent Frontiers in Ecology and Evolution paper (“A world without parasites: exploring the hidden ecology of infection“), and the similar and wonderful “soapbox” paper written by Bob Holt (“A World free of parasites and vectors: Would it be heaven, or would it be hell?“). Then there are these thought-provoking papers parasite conservation by Gomez et al. (“Parasite conservation, conservation medicine, and ecosystem health” and “Neglected wild life: Parasitic biodiversity as a conservation target“). Doughtery and colleagues also have a parasite conservation paper out in press that you might be interested in (“Paradigms for parasite conservation”).