Bacteria are microscopic single-cell organisms that are found in the air, inside and on our bodies, in the dirt, and everywhere in nature. There are both harmful and beneficial kinds. Some cause diseases, while others help our bodies function. For example, there are more than 400 types of bacteria live in the human digestive system. There are also kinds that are used to make medicines, and others that make foods like cheese and yogurt. (Might anyone know what kind of bacteria this is?)
More mentions of bacteria are in these videos.