I think that there are too many survivors outside of your city. They should be a rare thing to find, and if you don't get them quick enough they sohuld die.
Wow. Thanks for telling me that my game won't save over any of the other saved games if the slots are full. I just lost over an hours worth of effort, and even worse, it was the first time ever that I was winning the crazy challeng! I love this game but I can't even bring myself to play it again.. I tried so many times to beat the crazy challenge and when I finally had it, my game didn't save. :'(
Both are excellent game series :)