No bacteria exist everywhere in nature. They are in the soil, air, water, and the foods we eat. When they have nutrients (food), moisture, and favorable temperatures, they grow rapidly, increasing in numbers to the point where some types of bacteria can cause illness. Bacteria grow most rapidly in the range of temperatures between 40 and 140 f, some doubling in number in as little as 20 minutes. Some types will produce toxins that are not destroyed by cooking.
Pathogenic bacteria do not generally affect the taste, smell, or appearance of a food. In other words, one cannot tell that a food has been mishandled or is dangerous to eat. For example, food that has been left too long on the counter may be dangerous to eat, but could smell and look fine. If a food has been left in the danger zone between 40 and 140 f for more than 2 hours, discard it, even though it may look and smell good. Never taste a food to see if it is spoiled.