This paper examines whether urbanisation moderates the effect of carbon emissions on infectious diseases in Heavily Indebted Poor Countries (HIPCs) in Africa. The study is motivated by the public health risks created when rapid urban growth occurs alongside environmental degradation, weak sanitation systems, inadequate housing and limited health infrastructure. The analysis focuses on 28 HIPCs in Africa over the period 1990-2023 and uses three measures of infectious diseases: prevalence of HIV among the population aged 15-49, incidence of tuberculosis per 100,000 people and incidence of malaria per 1,000 population at risk. Carbon emissions are measured in metric tons per capita, while urbanisation is captured by urban population as a percentage of total population. The model incorporates gross domestic product per capita, current health expenditure and access to basic sanitation as control variables. Anchored on the Epidemiological Transition Theory and the Environmental Kuznets Curve hypothesis, the study estimates a fixed-effects panel regression model with an interaction term between urbanisation and carbon emissions. The results show that carbon emissions exert a significant positive effect on HIV prevalence, tuberculosis incidence and malaria incidence. Urbanisation also increases disease prevalence, although its direct effect on malaria is statistically weak. The study concludes that infectious-disease reduction in African HIPCs requires an integrated policy framework linking clean energy transition, sanitation investment, public health financing and urban disease surveillance. The paper contributes to the literature by showing that urbanisation is not only an independent determinant of infectious diseases but also a transmission channel through which environmental pollution worsens public health outcomes in fiscally constrained African economies.