The emergence of a multisystemic and invariably fatal syndrome characterised by facial deformity and lethargy in extinct in the wild Christmas Island endemic and invasive reptiles prompted an investigation into its aetiology. Knowledge pertaining to the ecology and impacts of the agent at the time of the outbreak were limited, threatening efforts to conserve the island’s endemic reptiles. The investigations described in this thesis therefore aimed to enrich understanding of the organism’s pathogenesis, pathophysiology, and epizootiology. Investigations into other agents with the potential to jeopardise conservation efforts were also explored. Following morphological characterisation, genomics revealed the agent was a novel species; Enterococcus lacertideformus, within the Enterococcus faecium clade, possessing genes encoding virulence determinants, resistance traits, and genes underpinning its biofilm phenotype and failure to cultivate in-vitro. Efforts to combat E. lacertideformus were also explored. Pharmacokinetics of enrofloxacin in-vitro showed that 10 mg/kg oral enrofloxacin likely achieved sufficient plasma concentrations. An experimental infection model indicated that the clinical course of E. lacertideformus depended on the inoculation route, and transmission likely occurred through direct contact. Following infection, an in-vivo therapeutic trial showed that enrofloxacin and amoxicillin-clavulanic acid were prospective candidates for treatment. Additionally, health surveillance and metagenomics uncovered disease processes and parasites, and two novel papillomaviruses, respectively, in Christmas Island and Cocos Islands reptiles. In summary, this thesis contributes substantial knowledge regarding the bacterium and its host interactions, while also providing foundational health data on reptile populations. Ultimately, this research guides management strategies and conservation opportunities and informs treatment protocols to combat E. lacertideformus.