The illustration shows how this kind of "plague" spreads: S stands for still-normal people, R for the dead, and Z, naturally, for zombies. An ordinary person can easily end up in category Z or R, and from R soon wind up in Z as well. There is no way back: this model works in only one direction, and very effectively at that. According to Canadian researchers' calculations, if a zombie infection begins in a city with a population of 500,000, within 3 days the number of zombies will exceed the number of normal people. In that case, there is only one piece of advice: show no mercy. source