This thesis presents studies conducted to investigate an outbreak of porcine high fever disease (PHFD) in a small area of Vietnam, in terms of mortality, morbidity, spatial transmission between herds, and risk factors for the disease. This is a severe disease with very high mortality in all age groups which has been considered to be caused by highly pathogenic porcine reproductive and respiratory syndrome (PRRS) virus strains. The focus of the thesis then shifted; to the investigation of within-herd transmission of PRRS virus (PRRSV) infection in commercial herds typically present in Ontario; to the evaluation of commonly used control strategies; and to the estimation of sensitivity and specificity of the PCR test used in surveillance of PRRSV. During our investigation of a PHFD outbreak, it was found that 33.4% of households were cases, and the mortality in these cases was 24.3%, 22.8%, and 6.7% in sows, suckling-nursery pigs, and finishing pigs, respectively. The spatial spread of the disease in the area was very limited, whereas introduction of pigs into a farm before the outbreak was identified as a risk factor. Moreover, it was also found that raising ducks in proximity to pigs and feeding of water green crop to pigs increased the risk for PHFD. For within-herd dynamics of PRRSV infection, the basic reproductive number (Ro) for PRRSV and duration of detectable maternal antibodies (m) in suckling and nursery pigs was estimated. Ro was found to be high (Ro=9.76 ) and m was short (m=3 weeks). The results of mathematical modeling suggested that it is possible to eliminate PRRSV infection from a herd by using herd closure or mass immunization. However, duration of sow immunity, and efficacy of immunization could play a critical role in this result. Finally, our study found that the sensitivity of tissue PCR is higher than the sensitivity of serum PCR and the likelihood of detecting the virus in tissue was higher in pigs with dyspnea or rough hair coat, but lower in lame pigs. This finding can help to increase the sensitivity of risk-based surveillance programs.