Things to improve:
- Sentences. Most, if not all of them, are simple sentences. It's good to have a few simple sentences here and there to speed things up, but try using some compound/complex sentences.
- Pacing. Like other people said, stuff was going fine one second but the next everything was being eaten. Try to build on the story/characters/etc. more until you jump into the action.
- Tense. You're using past and present but you're only supposed to use one, unless you're doing the kind of thing where they go and recap history and switch tenses, which you're not.
- Topic. At the moment, it seems like your typical zombie-apocalypse novel. It's not a bad topic, it's just so overused. Maybe you could add a twist to it.
That's about all my advice. Yay, I feel like a teacher
