Four synchronous epidemics of measles and scarlet fever are observed in the historical data collected by colonial authorities in Victoria, Australia from 1853 to 1876, suggesting some sort of synergistic relationship between the two diseases. While epidemics of measles, as recorded by the colonial record keepers, still occurred during the remainder of the study period (until 1916), no further epidemics of scarlet fever occurred after 1876. This is suggestive of a change in Victoria's disease ecology in the late 1870s. After analysis of the historical data for potential artifactual cases, it does not appear to be the result of confusion in diagnosis and changes in case definitions do not appear to have affected reporting of the causes of death. We conclude that the most likely explanation for the observed pattern is an epidemic synergy that ended after the 1876 epidemic. We hypothesize that this synergistic relationship between measles and scarlet fever in mid-nineteenth-century Victoria ended due to a shift in the dominant group A streptococci (GAS) M-type or the loss of a GAS bacteriophage. Support for this hypothesis comes from observations that other diagnoses associated with group A strep infections also changed their mortality profiles during the 1870s, particularly Bright's disease, a possible descriptor of post-streptococcal glomerulonephritis. We situate the emergence and end of this pattern within the demographic and socioeconomic conditions of the Victorian gold-mining boom in the 1850s to 1870s and postboom changes in fertility, mortality, and housing infrastructure, highlighting the importance of social conditions in disease evolution.