The emergence of infectious diseases has become an urgent concern for human and wildlife populations alike. Malaria parasites (genus Plasmodium), which are transmitted by mosquitoes, are major pathogenic disease agents in human and wildlife populations with numerous non-exclusive factors, including climate change and range shifts, acting to increase their threat. The malaria parasites that infect the Common Loon (Gavia immer) serve as an ideal system for studying emerging infectious disease threats with warming temperatures as the Common Loon is a northerly distributed, immunologically and evolutionary naïve species and of conservation concern across parts of its breeding range. I investigated the geographic and temporal distribution and diversity of Plasmodium parasites that infect Common Loons across the southern edge of their breeding range. In addition, I surveyed a diversity of mosquito species from the northeastern US to identify the vectors of the malaria parasites of birds including loons. Through broad collaboration samples were obtained from 461 Common Loons between 2014 and 2021 and from 2,867 ornithophilic mosquitoes of nine species. To detect Plasmodium parasites, Common Loons and mosquitoes were screened by nested PCR with each positive sample sequenced at the mitochondrial cytochrome b gene to identify parasites to lineages representative of species. Overall, 34 (7.4%) of sampled Common Loons were found infected with ten different Plasmodium lineages, including one novel lineage that has not been previously isolated and numerous generalist lineages known to infect a broad range of avian host species. Plasmodium lineages were found to demonstrate slight longitudinal structure across sampling regions, perhaps due to varying parasite-mosquito associations, host genetics, and/or host density, or sampling efforts. Plasmodium lineage diversity and infection prevalence lacked latitudinal or temporal structure at all sites except New Hampshire and Maine, where Plasmodium diversity increased through time. I isolated 12 Plasmodium lineages from four ornithophilic mosquito species screened including Culex pipiens, Culex restuans, Culiseta melanura, and Culiseta morsitans. Three specialist lineages were isolated from Culex species, suggesting that mosquito habitat preferences may influence parasite specificity. Results from this study elucidate the host-vector-parasite dynamics underlying parasite spillover and shed light on the importance of continued surveillance for Plasmodium parasites in both vertebrate and vector hosts to minimize future disease risks.