Sometimes it is difficult to tell whether you have influenza or some other viral illness. Many other respiratory viruses, including common cold viruses (rhinoviruses), may mimic the symptoms of a minor case of the flu.

Conditions that people mistake for the flu include:

• A common cold.
• Bacterial infections, such as strep throat, bronchitis, or pneumonia.
• Infectious mononucleosis.
• Stomach flu symptoms (gastroenteritis).

Viral infections such as a cold or flu tend to cause widespread symptoms, such as fever, body aches, a headache, and a sore throat. Bacterial infections tend to have specific symptoms involving only one system, such as a fever and sore throat only.

The flu makes you feel much worse than the common cold. Symptoms often include fever and cough within the first 48 hours of the illness.