In short, if you haven't seen it, watch 28 Days Later and 28 Weeks Later. It's a film about a virus that turns people into bloodthirsty "zombies," quite fast "zombies." Unlike many other similar films, this one is more realistic; it's not like Resident Evil, where one person single-handedly destroys hordes of zombies))
The first part is probably more interesting than the second, although the second is more... or rather, less cliché than other similar films. You'll understand when you watch it.