Symptoms of flu
Influenza (flu) is a contagious respiratory illness. It can cause mild to severe illness and even death. The flu is different from a cold. Flu symptoms come on suddenly and include⁹:

• Fever
• Headache
• Tiredness
• Dry cough
• Sore throat
• Nasal congestion
• Body aches
• Sometimes nausea, vomiting, and diarrhea

Differential diagnosis for flu

The symptoms of cold and flu, when considered all together, allow an empiric differential diagnosis.¹⁰ However a definitive diagnosis of symptoms identified as flu requires laboratory testing.

Learn about testing for influenza