Infections and other illnesses release stress hormones that can push blood sugar up, sometimes substantially, even when your appetite drops. This is why blood sugar may rise during a cold, flu, or stomach bug.
General sick-day principles include checking your blood sugar more frequently, drinking plenty of sugar-free fluids to stay hydrated, and continuing your usual medicines unless your clinician has advised changes. People with type 1 diabetes are usually advised to check for ketones when unwell or when sugar is high.
Seek urgent care if you have repeated vomiting or cannot keep fluids down, signs of dehydration, persistently high readings, ketones, deep or rapid breathing, or confusion or drowsiness, as these can signal a dangerous emergency. Agree a personalised sick-day plan with your care team in advance, and never stop insulin without medical advice.