We report the case of a 97-year-old woman who had a prolonged hospital admission for the treatment of right-sided heart failure. During her stay she experienced a rapid deterioration, characterised by shortness of breath, cardiovascular compromise and a hot, red, swollen calf. Post-mortem examination demonstrated that this was caused by necrotising fasciitis due to Serratia marcescens as a single pathogen. This is only the second reported case of this condition in the absence of diabetes or immunosuppression, and clinical deterioration was much more rapid. The case underlines the importance of circumspection and regular review in the diagnosis of the elderly patient. It reminds us that these patients should be viewed as functionally immunosuppressed, and that some or all of the haematological markers of infection can be absent even in severe disease.