Most people recover from flu at home, but certain signs mean you should get medical help straight away. In adults, these include difficulty breathing or shortness of breath, persistent chest or abdominal pain or pressure, ongoing dizziness or confusion, seizures, severe weakness, and not passing urine (a sign of dehydration).
A particularly important pattern is flu symptoms that improve and then return with a worse fever or cough, which can signal a complication such as pneumonia.
In children, watch for fast or laboured breathing, bluish lips or face, ribs pulling in with each breath, no tears when crying, severe drowsiness, or a fever with a rash. Call your local emergency number or go to the nearest emergency department for any of these. Do not wait.