Training after a flu bug?

Personally, I would go with listening to the body!  My general rule of thumb with viruses is that I won't do any training unless I'm feeling okay (normal energy level, no below-the-chest symptoms for colds, no gut symptoms) with normal non-workout activity, and I won't do anything hard until I've felt okay for a couple of easy workouts. I definitely wouldn't do anything like "boot camp" the first day back. It's too easy to relapse. There are rare but real cases where someone got heart damage by pushing too hard while still infected with common viral illnesses.  It happened to a good friend of mine in her early 30's, and she was left with permanent heart damage after a minor bout with the flu. Not to scare you, but boot camp will still be there next time. If you're feeling great, I'd do an easy run, swim, or bike.

1.  That's not the flu.

2.  Train when you feel ready.  Don't overcomplicate things.

From a running background--they always say you can run with a head cold, but don't mess with a chest cold, and anything in the tummy you probably don't want to run anyway!

I'd go to the boot camp but tell the trainer you're getting over a bug, and then just listen to your body.  You'll probably start feeling tired more easily.  When you do, rest a minute.  If you don't make it all the way through, then at least you tried.

The only other thing I'd say is be really careful of your hydration--drink plenty.  It helps you heal as well as train