After adding the ability to infect I've decided to have an option on the zombie settings screen to choose whether you want to infect humans over time or have them immediately be zombies.
So now you can have combinations of the two to start your simulations! You could pick a specific person to be "patient zero", or have infection take a very long time while there are still some zombies immediately to start, it's up to you!
As always, thanks for playing everybody!
Changed files in this update