Your respiratory illness might be the flu if you have fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and fatigue. Some people may have vomiting and diarrhea. People may be infected with the flu and have respiratory symptoms without a fever. Flu viruses usually cause the most illness during the colder months of the year. However, influenza can also occur outside of the typical flu season. In addition, other viruses can also cause respiratory illness similar to the flu. So, it is impossible to tell for sure if you have the flu based on symptoms alone. If your doctor needs to know for sure whether you have the flu, there are laboratory tests that can be done.