Leishmaniasis is a deadly protozoan disease endemic in more than 98 countries placing more than 1 billion people at risk and killing more than 40,000 annually. My thesis aims to understand further the epidemiology of leishmaniasis, the influence of tick borne disease co-infections on progression of disease in the U.S. and Brazil, and the impact of vertical transmission. This is done utilizing a dog model, Bayesian and frequentist statistics, risk factor analysis, and active/passive surveillance.