Fun game, I enjoyed the feeling of slowly building up a zombie horde around me to take people down. The amount of levels was nice, the particles looked great when infecting someone.
If you wanted to improve, I suggest having bullets take out only one zombie instead of just going through. It's a little frustrating to have one bullet take out half of my pack and kill me too.
I also suggest adding in some UI scaling, everything came out kinda small on my computer.
Overall, fun game!