Bacteria reproduce exponentially, meaning one bacterium becomes two, two become four, four become eight, and so on. Because of this, bacteria populations can double in four to twenty minutes, giving bacteria the ability to form millions of cells in as little as a few hours.
Under perfect conditions, one bacterium cell can theoretically multiply to one billion individual cells in only ten hours. Self-reproducing bacterium's success is in large part due to the fact they reproduce so rapidly and are very hardy survivors. Contrary to popular belief, freezing food does not kill bacteria, it only slows their growth. So once food is again heated up, the bacteria immediately begin reproducing at their regular rate.