Luckily Mira Grant also made a 0.5 book that is all about the virus, the first zombies and more.
This book was a bit confusing, with all the biological terms and such, but I still loved it.
Also the fact that we have about 5+ characters and they keep switching between them (if they are still alive) was a bit annoying, since it took me at least a page to find out who was talking now.
Other than that, I would really recommend this to everyone. The first zombie attacks were awesome to read about. How slowly panic took over.