While you wait for possible (or not) implementation ... The magic number for Walkers is 40 ... so if your Walkers on the board are at or near 40, then the zombie apocalypse has arrived. The magic number for Runners is 16 ... so if your Runners are at or near 16, then they may suddenly leap 4 or more squares into your square and attack you. I believe the magic number for Fattys is 16, so similar to Runners ... but with the added caveat that if a Fatty can not spawn and all the Fatties get activated, then that ALSO activates ALL Walkers to freak out on you and charge you and attack you and act like Runners ...
I hope that helps ...
And the biggest cure for this phenomenae is LOTS and LOTS of molotovs to take down Walker/Runner/Fatty population on the board ASAP ... and also reigning in the Wanda/Amy types of players that like to run around opening doors when there is no reason to (and very good reasons not to) open doors.