One of your patients visited your clinic because she is experiencing the following symptoms:

Severe aches in muscles and joints
Pain around the eyes
Weakness
Warm, flushed skin
Headache
Dry cough
As an experienced doctor, you quickly recognize that this patient has influenza (flu). You prescribe rest, plenty of fluids and medicine to relieve the symptoms. You tell the patient, she will probably feel bad for about a week to 10 days.

A month later this same patient returns to your office with the same symptoms as before. The patient claims that the flu came back and that she must have never fully recovered.

As the patient's physician, how would you most likely explain this?