This thesis reports a study into aspects of the discourse concerning the measles, mumps and rubella (MMR) triple vaccine. The aim of the thesis is to contribute to knowledge about the ways in which debates about science are enacted in the public sphere. The study uses a corpus-assisted discourse studies (CADS) approach to examine key lexico-grammatical patterns in the JABS corpus, a corpus of texts gathered from the website of the vaccine-critical Justice Awareness and Basic Support (JABS) group. The aim of the study is to discover how participants on the JABS website discussion forum draw on discursive resources to achieve their rhetorical goals. Comparison is made with the typical lexico-grammatical patterns in the NHSvax corpus, a corpus comprising texts from NHS immunization websites. The study finds that, although there are several areas of similarity between the two corpora, the JABS corpus data contains greater evidence of evaluative lexis, a higher frequency of nouns which express evaluations of the status of discursive objects. These resources are used to reformulate and reframe propositions which originate in the medical-scientific domain. Narratives of vaccine damage are also frequently used to express warrants for expertise.