Opossums and cat fleas have been epidemiologically linked to flea-borne rickettsial disease transmission in residential backyards of Orange County, California. In 2013, a study was initiated to better elucidate the life history of opossums and their role as vectors of disease and hosts for both internal and external parasites. The study population consisted of adult opossums collected year-round from flea-borne rickettsial disease exposure sites, and moribund opossums submitted by wildlife rehabilitators in Orange County. Carcasses were examined for ectoparasites and necropsied, which included the removal and collection of endoparasites, organ tissues, feces, and urine. Reproductive life history data suggested one brood of young per year, with an average litter size of 7 (n = 9, range 2-11). Average adult weight was 2.49 kg (range 1.30-4.41 kg). Cat fleas were present on each opossum with an average of 96 fleas per opossum (n = 82, range 2-725). Thirty of 33 cat flea pools tested PCR-positive for one of the following bacteria: Rickettsia felis (53%), R. typhi (3%), the R. felis-like organisms, Candidatus Rickettsia senegalensis (28%) and Ca. Rickettsia asemboensis (3%), or Bartonella vinsonii subsp. arupensis (1.5%). Sticktight fleas (Echidnophaga gallinacea), the only other flea detected, were present on less than 6% of opossums, and ticks were not detected on any carcasses (n = 83). Endoparasitic nematodes Cruzia americana and Physaloptera turgida were present in each stomach and cecum, and Didelphostrongylus hayesior or Heterstongylus heterostrongylus was noted in lung samples of opossums (n = 83). Salmonella spp. were detected in 52% of fecal samples (n = 50), with subsequent typing of strains indicating the presence of human pathogens in all but three of the samples (n = 26). Blood and spleen samples were negative for Bartonella spp., Brucella spp., and Yersinia pestis (n = 33). Sera were negative for Leptospira-specific antibodies and Leptospira DNA was not detected in urine (n = 83). Results from this multi-agency study show that the presence of opossums in the backyard environment put Orange County residents and their pets at risk of flea-borne bartonella and rickettsial diseases and salmonellosis.