Bacteria are present in almost every environment, e.g. in the soil, in the air, in animals and plants. They are found all over our bodies e.g. on our skin, in our mouth and in our gut. Most are harmless and are even beneficial. Bacteria that live in our gut help us to digest our food. See the What makes me ill section for more details. The American Society for Microbiology has a website that explains all about where bacteria live.