A fundamental question in the study of pathogenic microorganisms is how host specificity and virulence are determined on a molecular level. My research focuses on addressing these questions using the human pathogen Toxoplasma gondii as a model system. Toxoplasma is an obligate intracellular parasite that is closely related to other pathogens with a significant impact on human health, including the causative agents of malaria and cryptosporidiosis. However, a number of intrinsic features of T. gondii differentiate it from these other organisms, making it a particularly robust system in which to study the evolution of parasitism. Toxoplasma has an extremely broad host range, being capable of infecting all mammalian and bird species. Recent identification of parasite effector proteins that determine virulence in the mouse model have been made. One such protein, ROP18, is a secreted kinase that can transform a benign Toxoplasma strain into a lethal one. Understanding the mechanism of action of ROP18, as well as other key parasite effectors, should yield new insights into the evolution of the host-pathogen relationship on the molecular level, and provide a basis for the development of more effective treatment regimens in human cases of toxoplasmosis.