Survive the Zombie Apocalypse

.. which is nigh impossible to avoid when you are farming leaves. I -hate- this event. There really should be a way to opt out of it.

It is annoying.
However, you can just stock up on vaccine (kill a Vaccine Wagon, they give 10 basic vaccine and 1-2 of every other kind but Source) and use one every half hour while farming leaves. Or whenever the "infected" icon looks like it's getting full.