There are no random game overs people. When the timer reaches zero you've lost, it's that simple. Once the objective of a level is done the timer stops ticking so you don't have to worry about the life span of your zombies being too long. Level 1 can be cleared without even starting the timer if you hit 6 people with the zomBomb.
Damn this is hard. There's a certain level where my zomBomb doesn't seem to be effective enough to infect more than 1 -2 doods, even with a maxed out infection rate.