Oh finally! I got it! Thanks to the upgrading thing, I upgraded my zombies to last more time and increase the chances of infection. The zombies got to the people and killed them, then THEY became zombies and just kept going on until they killed all humans.
I beat this in 355 days on the game by the way. Just thought I should mention that.
This game is fun. It makes me sad how almost everyone is saying this game sucks. It's really fun in my opinion. And somebody worked hard to make this game...would it kill you to give them a bit of respect?