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Climate conditions, such as ambient temperatures, play a crucial role in infants' vulnerability to infectious diseases. However, little is known about how climate conditions, such as temperatures and seasonality, affect infectious disease mortality among infants in high mortality settings. The aim of our study was to investigate the association between cause-specific infant mortality and ambient temperatures and seasonality. We applied a retrospective study design using parish register data from Sweden covering the 1868–1892 period in combination with daily temperature data. Mortality due to water- and foodborne diseases, airborne infectious diseases and other causes was modelled as a function of temperature exposure in the previous 14 days using distributed lagged non-linear models. We found that airborne infectious diseasemortality was not related to cold temperatures, but rather to seasonality. The summer peaks in mortality due to water- and foodborne infections were associated with high temperatures, and not with seasonality. The increased vulnerability of infants to infectious diseases at high temperatures is a significant future risk, given that global temperatures are projected to rise in the coming decades.