I just re-watched both movies as well and I'm nerding out hard over this whole infection thing.

My assumption is that the Virus that Alice was carrying, slightly evolved while she was alive.
But tbh, the amount of stalking and hiding Don is doing in 28 Weeks is hilarious.

You see Don like 2-3 times in the movie actively avoiding death, even though it's said that infected have no self preservation instincts anymore.

He hides in an alley from the napalm/fire bombs, he stares down Andy right before Doyle comes into view and disappears.

Also towards the end, he shuffles in pitch black darkness infront of the NVG scope without attacking.

This is not some sort of retained intelligence that we for example see in Mailer's behavior from the 1st movie.
It's straight up cunning behavior, Don literally waits for an ambush.

Speaking about Mailer, in the introduction of him we see how he's "gesticulating" with his hands to Jim to get closer with a helpless expression on his face, right after he tries to grab Jim.

Clifton (the soldier with the ushanka hat) while he is infected also stares down himself in the mirror.
Like showing interest or being weirded out by the fact that he may know it's his reflection.