When people become unwell with a minor illness like influenza, a urinary tract infection or gastric upset, they are likely to lose their appetite, and low energy levels are common. Most people just want to stay in bed, take the relevant medication, sleep, and hope the discomfort recedes – which it normally does. However, for a person with diabetes, the impact of such an intercurrent illness might not be so innocuous: without careful management, rising glucose levels can lead to severe decompensations – and in some cases, a life-threatening medical emergency. Penny Morris and Lesley Campbell look at two common sick day scenarios, and outline some important advice for people with diabetes and their healthcare providers.