The work described in this thesis was concerned with identifying the prevalence and risk factors associated with colonisation by the intestinal spirochaete Brachyspira pilosicoli in: humans: long term residents of Perth, Western Australia (WA) and Indonesians either living temporarily in Perth or as long term residents in urban and rural areas of Bali, Indonesia, animals: domestic animals including alpacas, birds, cattle, cats, chickens, dogs, doves, ducks, goats, horses, pigs, and sheep (housed at a wide variety of places around Perth), and a range of wild animals housed in various Zoos and wildlife centres in WA. This study shows that for humans: • Brachyspira pilosicoli was significantly more prevalent in Indonesians of all sub groups, be they temporary residents of Perth (9.4% - 216 faecal samples from 180 individuals), or long term residents of Indonesia (12.6% - 992 faecal samples from 617 individuals) compared with long term residents of Australia living mainly in Perth (0.2% of 766 sampled), even in those with gastrointestinal complaints. This suggests a relationship between a high prevalence of B. pilosicoli and living in Indonesia; • In Bali, B. pilosicoli was significantly more prevalent in the impoverished urban area of Sesetan (20.3-23.4%) where the husbandry of pigs is poor and effluent treatment is non-existent compared to four traditional farming villages (Badung, Karang Suwung, Melinggih, Payangan Desa) (3.3-22.6%). In the latter villages effluent and drainage is better and there is less likely to be contamination of drinking water • There was no significant association between the presence of B. pilosicoli and the presence of clinical symptoms including headaches, abdominal pains, diarrhoea, joint/muscular pain and constipation. • Amongst Indonesians living in Indonesia, there was no significant difference in the prevalence of B. pilosicoli between people with and without contact with animals and between farmers and other occupational groups. • Indonesians visiting Perth who were positive for B. pilosicoli originated from nine cities and five main islands in Indonesia. This suggests that B. pilosicoli is endemic throughout Indonesia. • Strain typing of isolates of B. pilosicoli showed that they were genetically heterogenous and did not show any consistent pattern with respect to geographical location, family of origin or disease status. Isolates from the same individual were sometimes unrelated, suggesting the probability of re-infection with another strain between the samplings. • Some households (~7%) had more than one member positive for B. pilosicoli. Strain analysis suggested transmission between family members, and this could be due to either faecal-oral transmission, or from a common external source, such as contaminated water. • B. pilosicoli was cultured from only 0.2% of Australians. This low prevalence may be a result of little or no exposure to B. pilosicoli due to good personal hygiene and environmental sanitation. • B. pilosicoli strain H1b and H171 that were isolated from healthy Indonesians were able to colonise mice and day-old chickens, and induced clinical signs of pasty faeces in the latter. Histological sections showed mild typhlitis and typical end-on attachment of B. pilosicoli to the caecal epithelial mucosa of the chickens. This finding suggests that the human isolates had pathogenic potential. This study showed that for animals investigated: • Intestinal spirochaetes were cultured from 46.4% (13/28) of bilbies with 14.3% (4/28) positive for B. pilosicoli. Spirochaetes were also cultured from the faeces of two Western Barred bandicoots and one (1.2%) kangaroo. • Intestinal spirochaetes were not isolated from any alpacas, cattle, goats, horses, pigs, and sheep but were detected in 40.5% of ducks, 14.3% of chickens, 14.9% of ostriches and 1.5% of cats. • Few pets that are commonly kept in households (dogs, cats and aviary birds) were colonised, suggesting that they are not an important focus of B. pilosicoli infection in Australia.