The porcine proliferative enteropathies (PPE) include a variety of pathological conditions commonly described as separate entities: these include proliferative haemorrhagic enteropathy (PHE), necrotic enteritis (NE), regional ileitis (RI) and porcine intestinal adenomatosis (PIA). All of these pathological conditions are characterised by development of proliferation of the intestinal mucosa, particularly in the distal small intestine and proximal large intestine and the presence of curved, rod-shaped, intracellular bacteria, Lawsonia intracellularz‘s, in the apical cytoplasm of the epithelial cells. PPE can occur in all swine production systems and involve pigs of all age groups, however the more severe clinical from, PHE that causes higher mortality, predominantly affects adult pigs. In the endemic herds, pigs can become infected with L. intracellularis at different time points and shed bacteria in their faeces. Although factors that may play a role in the transmission of L. intracellularis infection have not been identified, spreading of the infection between pigs through the ingestion of contaminated faeces is considered the most important mode of transmission. Using an oral challenge model, this thesis examined how the number of ingested L. intracellularz's could affect the establishment, temporal pattern of infection, clinical disease and immune response to the infection in pigs. Susceptible pigs of all ages exposed to L. intracellularis develop infection and an immune response. They shed L. intracellularis in their faeces and, as the bacterium can not be grown in any artificial medium, the recent development of polymerase chain reaction (PCR) allows the detection of a current infection with L. intracellularis in live pigs by detecting L. intracellularis DNA in the contaminated faeces. Development of immune response to L. intracellularis in infected pigs by production of specific immunoglobulins including IgG, IgM, and IgA in the serum can be detected by the indirect immunofluorescent antibody test (IFAT). One of the aims of this thesis was to investigate the extent of the immunity to L. intracellularis in protecting previously exposed animals from re-infection. In—feed administration of antibiotics is widely practiced in pig production to control many bacterial infections including L. intracellularis as well as for growth promotion. It is important to assess the effect of different groups of antibiotics at different concentrations on the disease pattern and on the development of immunity to re-infection in pigs. Optimising the selection and concentration of some antibacterials can help to reduce the amount and extent of drug use while maintaining a good health status of a pig herd and improving profitability of pig production. As a part of this thesis, an investigation was conducted to determine if L. intracellularis existed in the system of commercial pig farms in the North of Vietnam. Some piggery management factors which may be associated with the occurrence of L. intracellularis infection in these pig herds were also identified and analyzed. The laboratory techniques developed in Australia such as faecal PCR and IFAT appeared to work well and to be useful as tools for diagnosis and survey of L. intracellularis under Vietnamese conditions.