My recommendation is to go and see it, if you can stand to watch a frightening and bloodthirsty film. It's a significant film IMHO, and it's an emotional trip. It's just the emotions aren't that pleasant: dread, horror and despair.
I went with my son and his mate to see it last night. There was one scene - the fulcrum of the film - where I had to put my fingers in my ears and my palms over my eyes for what seemed a long time. When I thought it was safe I took my hands away from my eyes, to find that my son was leaning forward with his hand between my eyes and the screen, in case I opened them too early. 'Not yet, mum' he said.
The film is by the director of 'Intacto' which is a damn good film, which you ought to look out for. It uses a wide palette of - I don't really know the technical term - different styles of film image. The issues raised are not clear cut, they explicate the tragedy rather than offer any resolution. The portrayal of the American army for instance, while pointed, is not simple or one-sided.
Why don't the infected attack each other? The answer of course is that it would short-circuit the story, and wreck the allegory. In 'real life' a virus of this kind would be self-limiting, and easier to contain for that reason.
I thought that the effects of the virus might include an inhibition against attacking other virus carriers (you could at a pinch imagine that such a trait could be engineered into it). This would mean that the 'typhoid Mary' plague-carriers would be able to survive in infected zones. Obviously this wasn't what happened, as Carlyle wouldn't have attacked his wife.
I think horror films take us back to our long prehistoric past, and remind us what it was like to live in a world of dangerous animals that ate people, and shared our caves and tundras. And they remind us that the point of parents, in a way, is to die so that the children survive, because that's how we lived for millions of years. I think the prehistoric dread is even more fundamental to the film than the Iraq allegory.