A screaming madman racing towards a couple of armed Soldiers and not one manage to harm him? I found that stupid. He wore plot armour for the entire Movie because he was spreading the infection and being the father-monster stalking his Children. I could not accept things unfolding so neatly like that because it had a realitic feel other times. Don was way too much of a "villain".

And manage to kill several armed Soldiers once he gets infected in the beginning.
Before the siren. Before anybody knew the infection was on the loose again.
After the siren, every trooper in the place was on edge and ready to shoot and ask questions later.
Till this thing broke out again, most of those soldiers maybe, maybe, saw tapes of these things.
I'd say this whole thing was fairly realistic. What bugs me after multiple viewings is the "shaky-cam". Who originated that? Darned Tony Scott?!
Oh, and Don being a "super infected"? I'd never thought much about his slightly differing behaviour. I think one thing that his behaviour pointed out (being the solitary infected, at first) was that he won't take on groups of normal folks. See, typically we've seen groups of infecteds chasing individuals or small groups of normals. But one thing we can deduce from Don's background appearance when Doyle was leading the survivors out of the facility, was that Don the Infected didn't charge them. Why? Because he was just one, versus 5-6 of them.
That right there could suggest that there's an inherent logic to their attack behaviour. Hmmm.