Code Zero wasn't as good as Patient Zero, but it's pretty close. I'd not read anything by Maberry since Patient Zero and when I saw that Code Zero was a "sequel," I jumped at the book. Unfortunately, there have been many Joe Ledger novels in between and a good bit of Code Zero references what must be missions from those previous books. Now, that's my fault. But I say this to encourage others that if you've not read those other books (which I'll now go back and do, frustratingly after the fact), you might want to.
While reading Code Zero, I kept getting the feeling I was reading Matthew Reilly. And that's a big compliment. (If you've not read Reilly, give him a try; his books are VERY fast-paced and a lot of fun!)
In Code Zero, we see a terrorist named Mother Night and her minions creating chaos all over the US. As readers, we soon learn who she is, although the rest of the DMS crew don't figure it out until much later. This is somewhat anticlimactic for the reader. I would also like to have gotten a little more character development from another antagonist as he was a little to cookie-cutter and shallow. Still, Code Zero is full of over-the-top action with scary, genetically-altered monsters in it.