Don. Robert Carlyle is good at creepy anyway, but him as an Infected was nightmarish, and that's not an easy thing to do. The scenes where he's first turned and what he does to Alice are horrifying, and the fact that he seems to retain some very basic reasoning just makes it worse. He's not a mindless, murderous creature like the Infected in the first movie—he's still very murderous, but he's got enough marbles left to actively hunt his children through London.
When residents in the center are put into a sort of 'quarantine room' at the first sign of the infection. It's a small room, and there are a lot of people. Making it very cramped and claustrophobic. Bad enough by itself... But then a single infected gets inside through the only entrance, and everyone in there realizes they have no way to escape.
Andy's nightmare near the beginning, in which his mother peels off her own face while staring straight at the camera.