My main objective is to produce and test predictive theory of the ecology and evolution of disease. Together with collaborators, my work focuses on micro-parasites and cancer cells that can attain enormous population sizes within their hosts and therefore are subject to within-host evolution and at larger biological scales, host-parasite co-evolution, and have made significant inroads to fundamental research questions and applications to medicine. We have shown notably how space and environment arbitrate disease ecology and evolution at different biological levels, and also explain evolutionary transitions from conflict to cooperation and from parasitism to mutualism, these latter themes producing a more general conceptual framework for how environment interacts with social evolution.