I am interested in determining how parasite fitness is shaped by interactions with host(s) and environment. Questions that motivate my research include: 1) How do biotic and abiotic factors regulate dynamics of parasite populations within and between hosts? 2) To what extent do parasites and their hosts vary naturally in response to these factors? 3) What are the genotypic and/or phenotypic mechanisms driving this variation? 4) Most anti-parasitic/pathogen drugs and vaccines seek to reduce parasite densities in the human or vector; can their efficacy also depend on biotic and abiotic factors? 5) Is parasite density (“quantity”) less important than “quality”, when, and why? 6) When is it more beneficial to “target” the host, rather than the parasite? 7) Can we leverage our understanding of the major drivers of parasite population dynamics to develop a deterministic basis for explaining and forecasting parasite emergence/re-emergence in dynamic environments? 8) Can we synthesize this knowledge to develop ‘evolution-proof’ approaches for control and elimination? I address these questions with Plasmodium parasites that cause malaria in humans and mice and are transmitted by Anopheles mosquito vectors.