This thesis presents studies on the epidemiology of vaccine preventable VIruS infections investigated by oral fluid testing. It involved studies of both rubella and hepatitis B virus infections by the detection of specific IgG in oral fluid and of rubella virus by PCR detection and genotyping of viral nucleic acid. To select an oral fluid collection device with optimal performance for these studies three (Orasure, Omni- SAL, Oracol) were compared. Each device collected oral fluid of sufficient quality for qualitative analysis of virus specific IgG, but DracoI was most acceptable to the subjects being tested. IgG capture ELISA (GACELISA) tests were developed for detection of rubella and anti-HBc specific IgG in oral fluid. Their sensitivity was less than that of corresponding serum ELiSAs, with sensitivity decreasing with increasing age of subjects. Whilst the performance of the rubella GACELISA was shown to be an improvement over the existing radioimmunoassay and was particularly sensitive with samples from paediatric populations, the performance of the anti-HBc GACELISA was not considered good enough for further use. The development of an RT-PCR assay targeting the El gene of rubella virus enabled the molecular epidemiology of rubella to be investigated using samples from the UK, China, Greece, and Brazil. By comparison to previously reported strains, the majority of strains were assigned to three banches of a phylogenetic tree. Those assigned to branches 1 (UK, Greece, China) and 2 (China) were not closely related to any previously reported strains. The timing of oral fluid collection and subsequent storage was critical for PCR detection. Ideally oral fluid samples should be collected within 14 days of the onset of symptoms and stored at least at -20°C prior to testing.