I think that I've only had a "real" flu once. It was quite awful -- running from both ends and not enough strength to make it to the bathroom without help. I've been sick other times with what most people call the flu, but those were pale shadows in comparison -- I might have been miserable, but that's the only time I was helpless. Still, it was intestinal -- from what I've read, flu strains that attack the lungs are the really bad ones, like the H5N1 avian flu that was reported as a potential pandemic.

Of course, if you go back to 1918 there's the "Spanish Flu" in that category. It was an amazing bug; it had a high mortality rate and was actually most effective against people with strong immune systems. Most flu only kills the very young and very old, but half of its victims were between 20 and 40. I saw a documentary on PBS where they said it struck down strong, healthy individuals so fast that they might have a light cough in the morning and be dead before sundown.