I'd be interested to hear the answer to this, too.
I don't catch too many colds, perhaps one or two a year it seems. I am not sure I have ever had the flu. People say "Oh, if you had the flu, you'd KNOW" but... either when I have the flu it's milder than other people, or else I've just had some real bad colds involving muscle aches, fever, etc. such as you indicated, but I generally have a fast recovery time, it only lasts maybe 2 or 3 days. I really have no way of knowing what exactly it is.
Some people always seem to have something or other. I don't worry myself too much when I'm around those sick people -- I mean, I don't worry about myself catching it.
I will say this, I can smell a virus. When somebody has a cold or whatnot, I can smell it when I walk into the room, first thing, and the closer I get to them, the stronger it is. This can be an indicator to me if I'm likely to catch their cold or not, too, somehow.
Last month, I was around somebody for several hours on a Friday night who was recovering from a bug. We danced and talked and were in close quarters, and he had that virus smell very strongly. I wasn't concerned until I left, and could still smell it strongly for a while, which meant I'd brought it with me. I had to sing an entire recital program on Sunday afternoon, in other words it was not a good time to catch a virus (because one of the first things affected is the voice) so I just knocked back a bunch of vitamin C and vitamin D all day, drank green tea and avoided sugar. I could tell it I was battling the virus - that it was still lingering around me - but that seemed to keep it at bay, and in the end I was able to sing nicely and never did come down with that virus.
On the other hand, just this past weekend I was very naughty and essentially didn't eat anything except (homemade) cookies and scones all weekend! I started to get some symptoms that are very similar to one of my colds, but it was really just a reaction to eating all that crap. I didn't have the virus smell/taste, and after not eating that crap today the symptoms are quickly dissipating.
The other thing I've noticed is that some people miss a LOT of school or work due to one health thing after another. Now, I'm happy to have overall good health, BUT at the same time, I'm jealous, since I am pretty much never ill enough to skip anything. Oh, I may have an annoying sniffle and be lethargic, but it's really not enough for me to justifiably stay home "sick."
One other thing -- you know the expression "feed a cold, starve a fever", well, I pretty much starve any illness I have, and I don't know if that makes a difference.
Those are just my thoughts on the cold vs. flu vs. ??? topic. Interested to read other thoughts!