Bacteria are very small creatures that are present all around us, within us, and almost everywhere. Bacteria are so small that they cannot be seen with the naked eye, but we can definitely see the influence of bacteria in our lives! For example, the bacteria that live in the gut help us to digest different substances; the bacteria that invade our food can ruin it; and some of the bacteria that enter our bodies might cause diseases–these are the bacteria that we want to fight.
To understand how we try to fight a community of billions of bacteria, first we need to understand how so many bacteria can be generated from one bacterium that enters the body. When bacteria are in the presence of food, for example inside the human body or in an animal's body, they eat and reproduce by cell division: one bacterium divides into two identical bacteria, those two bacteria eat and divide into four bacteria, those four bacteria divide into eight, and so on. Overall, the number of bacteria increases very quickly, at a rate that is called "exponential."

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