The raccoon roundworm, Baylisascaris procyonis, is a common intestinal nematode of raccoons (Procyon lotor) and occasionally domestic dogs. Infection with larval stages following ingestion of infectious eggs in feces of these definitive hosts is capable of causing fatal neural larva migrans in a broad variety of paratenic host species. Approximately 50 human cases are recognized and incidence may be increasing. However, many knowledge gaps exist in understanding the epidemiology and transmission of B. procyonis. The goal of this dissertation was to employ an interdisciplinary, One Health approach to investigating B. procyonis in human, wildlife, and domestic hosts, including 1.) risk of occupational exposure among wildlife rehabilitators, 2.) infection dynamics and survival among rodent hosts, 3.) developing serologic tests for diagnostic purposes, and 4.) the role of dogs as B. procyonis hosts. Among wildlife rehabilitators, 7% (24/327) had antibodies to Baylisascaris suggesting prior subclinical infection. Significant risk factors included region, B. procyonis prevalence in raccoons, and consistency of hand hygiene after contact with raccoons/their feces. A questionnaire on knowledge, attitudes and practices revealed that correct knowledge and attitudes depend on factors such as educational background and experience. Reported use of personal protective equipment and infection control by raccoon rehabilitators depended on similar factors. Detection of Baylisascaris in non-definitive hosts remains challenging, and serology is the only ante-mortem diagnostic tool available. A recombinant ES-antigen based ELISA was developed to investigate serologic responses among experimentally infected rodents, but it was not successfully adapted to human testing. Studies on tolerance and survival among Peromyscus spp. (deer mice) demonstrated species-level differences in infection dynamics which may influence parasite transmission and maintenance. Finally, studies on patent B. procyonis in dogs revealed aspects of epidemiology and infection biology. From a national reference laboratory database, Baylisascaris eggs were detected in 0.005% (504/9,487,672) of dogs. Experimental infections in dogs and raccoons revealed lower host competence in the domestic host versus the natural raccoon host. Only 2/12 dogs became infected compared to 12/12 raccoons, with longer prepatent periods and lower egg outputs among dogs. Collectively, these studies answer important questions on the transmission and expanded lifecycle of a high-consequence zoonosis.