Host-parasite interactions can shape ecosystems, from individual species to entire communities. These interactions are highly heterogeneous due to complex relationships between hosts, parasites, and their environment. Environmental effects on host-parasite interactions are especially important in systems built around ecosystem engineers as negative parasite effects can be amplified onto entire communities. Additionally, as our climate changes, many of the environmental factors that shape host-parasite interactions are predicted to change as well, making it critical for us to identify the factors that effect host-parasite dynamics. To better understand the relationships between hosts, parasites, and the environment, we used a combination of field and laboratory studies to evaluate the affects of tidal elevation, air temperature, and predation on interactions between the ecosystem engineer Crassostrea virginica (eastern oyster) and two of its most lethal parasites, Perkinsus marinus and Haplosporidium nelsoni. The probability and intensity of parasite infections, and the probability of co-infection by both parasites, was significantly higher at higher (intertidal) than lower (subtidal) tidal elevations. This demonstrates that environmental factors can shape host-parasite interactions across small spatial scales and physical gradients. Because environmental conditions can fluctuate rapidly over short periods of time in the intertidal, we assessed the effects of increasing air temperature during air-exposure on oyster survival and immune response, and P. marinus infection intensity. We found that the parasite may benefit from increases in air temperature until its optimal temperature is reached, but the host has a higher capacity to survive at temperatures above the parasites optimum. However, if temperatures exceed the host threshold, then neither species will benefit. Lastly, we evaluated how biotic factors (predation) affect oyster-parasite interactions. Though its been documented that predators can affect host-parasite interactions in other systems, we found that predators do not affect oyster-parasite interactions. Through our research, we have increased our understanding of the relationship between the environment and host-parasite interactions by identifying several environmental factors that influence oyster-parasite interactions. This knowledge will be beneficial for creating research and management initiatives in the future that protect not just this engineering species but also the ecosystems that it creates and physical gradients. Because environmental conditions can fluctuate rapidly over short periods of time in the intertidal, we assessed the effects of increasing air temperature during air-exposure on oyster survival and immune response, and P. marinus infection intensity. We found that the parasite may benefit from increases in air temperature until its optimal temperature is reached, but the host has a higher capacity to survive at temperatures above the parasites optimum. However, if temperatures exceed the host threshold, then neither species will benefit. Lastly, we evaluated how biotic factors (predation) affect oyster-parasite interactions. Though its been documented that predators can affect host-parasite interactions in other systems, we found that predators do not affect oyster-parasite interactions. Through our research, we have increased our understanding of the relationship between the environment and host-parasite interactions by identifying several environmental factors that influence oyster-parasite interactions. This knowledge will be beneficial for creating research and management initiatives in the future that protect not just this engineering species but also the ecosystems that it creates.