A clearly well thought out and executed concept - and quite an enjoyable one too, as it was extremely satisfying running around and spreading the infection. The pixel graphics also fit the game really well, and are pretty good considering you only had two days to make the game. You fit the theme and prerequisite perfectly, as zombies are so much more than just your friend; they are your key to winning. The music was also funny and fit the game well.
I'm not sure what the random image of Markiplier in the middle was there for.... I understand the humor but feel that it kills the pixel vibe a bit. I also think that while this game was fun to play once, it lacks the replayability that's essential for a strong game that'll keep players returning. There's also no way to lose, so there's not much of a reason to strategize and try too hard to reach the goal.
Nonetheless still really fun to play and a fun concept! Nice work.