The army turns up, trying to help, but as they try and contain the contagion, they start to separate the infected, which is when the pandemonium really starts. I won't go into too much detail, but if you like "the Government as a Suspect" horror, you'll like this film.
The film itself was pretty decent, if you like that sort of thing, but I remember a film from the mid 1980s that told a similar story, so I'd pretty much guessed what would happen before the start.
I'm not sure how much of a horror geek I am when I can work out what the story is going to be from a short trailer I see in the cinema.