Infection disease is an increasing threat in the ocean. As the oceans continue to warm, outbreaks of disease in marine organisms and the risk they pose are projected to increase. Outbreaks in keystone and foundation species can change marine community structure and biodiversity permanently. Understanding the factors that lead to outbreaks of disease in these species and the mechanisms of resilience in host populations is of critical importance. My doctoral research aims to expand our understanding of infectious disease outbreaks in the ocean through focus on two disease systems: sea star wasting disease (SSWD) in the keystone species Pisaster ochraceus (Ochre star), thought to be caused by an unidentified microbe, and seagrass wasting disease (SWD) in the foundation species Zostera marina (Z. marina), caused by the protist Labyrinthula zosterae (L. zosterae). Both are widespread, temperature sensitive diseases of temperate nearshore waters which impact ecologically valuable species and have led to devastating population declines and habitat change. Through observational studies, field experiments, and controlled lab experiments I investigated: 1) the role of temperature and demography on Ochre star mortality during and following the 2014 SSWD epizootic in an observational field study and lab experiment, 2) transmission of SWD through the water in the field and the infectious dose of L. zosterae, and 3) pathogenesis of SWD over the course of infection and the relationship between SWD visual signs and pathogen load in Z. marina. In the first study, SSWD disease risk was correlated with both ochre star size and water temperature, resulted in shifts in population size structure to smaller animals as adult populations fell to one quarter of pre-outbreak abundances. The results of the field experiment on SWD showed L. zosterae is transmissible through water without direct contact with infected plants. The infectious dose of L. zosterae through waterborne exposure was 6 cells mL-1 and was not impacted by temperature. In the pathogenesis experiments, I found a complex relationship between the pathogen and host response which can be used to distinguish substages of pathogenesis. Both my findings and the methods I developed provide valuable resources for future work on the ecology and host-pathogen relationships of marine disease outbreaks.