I've had the 'flu, followed by a couple of weeks of chesty coughing which felt like a secondary infection. 23 days after starting with the 'flu on 12/12, I woke up and knew it had gone, but I was exhausted and am still tired, sleeping for at least an hour in the afternoons. My appetite is just about coming back, but not what it was. Others report much the same.
I was just on the point of ringing the Dr. when I started to feel that life might just carry on, so I didn't. It wouldn't hurt to ring.