Host-pathogen interactions can be influenced by environmental conditions and interactions with other hosts, either directly through the modification of pathogen transmission or development inside of hosts, or indirectly by influencing host or pathogen demography, survival, or functional traits. In this dissertation, I investigate several environmental (e.g. nitrate) and ecological (e.g., competition) factors that could influence host-pathogen interactions, using a model system of Daphnia species infected by an environmentally-transmitted fungal pathogen. I use this system to examine 1) the effect of nitrate pollution on host demography, pathogen survival, and infection dynamics, 2) how host-pathogen interactions respond to variable environments, 3) if a critical host density is present, and predictable, 4) how competition with a non-susceptible competitor influences epidemic dynamics, and 5) how pathogen exposure and infection influences host fitness for a number of host species differing in susceptibility.