The Medical-Officer-of-Health (MOH) report emerged from the need to articulate for government officials matters related to poor sanitary conditions in industrialised 19th-century Britain. If-conditionals were used in this genre of case reporting to gain wider acceptance of MOH claims. The twofold aim of this research is to investigate the macro-functions of if-conditionals and their sociohistorical meanings, and to analyse how participants in MOH discourse are represented through if-constructs. A historical discourse analysis of the semantic functionality of if-conditionals was conducted on a diachronic corpus of MOH reports (mid-nineteenth century – early twentieth century) which detail the spread of smallpox infection. The analysis accounts for the context-sensitive functionalities of if on the semantic level supported by corpus-assisted discourse analysis. Results highlight the semantic shift in the macro-functional use of if across the two subcorpora based on form-to-function mapping, and further underline how the if-operator conveys dynamic representational meaning in the diachronic evolution of MOH discourse.