Bacteria are small single-celled organisms. Bacteria are found almost everywhere on Earth and are vital to the planet's ecosystems. Some species can live under extreme conditions of temperature and pressure. The human body is full of bacteria, and in fact is estimated to contain more bacterial cells than human cells. Bacteria are disease-causing, microscopic, single-celled organisms with prokaryotic cell structures. They do not have membrane-bound organelles, including a true nucleus. These tiny, single-celled organisms—bacteria—are among the most ancient, numerous, and influential forms of life on Earth. They are so small that millions can live in a single drop of water, yet their impact spans the globe, from shaping ecosystems to influencing human health and disease. What are bacteria? Bacteria are microscopic living organisms that have only one cell. The word for just one is “bacterium.” Millions (if not billions) of different types of bacteria can be found all over the world, including in your body. They’re on your skin and in your airways and mouth. Learn what bacteria are, where they live, how they affect human health, and why some bacteria are beneficial while others can cause illness. Bacteria are microscopic, single-celled organisms found almost everywhere on Earth. They live in soil, water, air, food, and inside and on the bodies of plants and animals. Some cause disease, but most do not. Many bacteria are essential to ecosystems, food production, agriculture, and human health. Bacteria are microscopic infectious agents that have a long history of infecting humans, but they also play a vital role in supporting human health. Bacteria are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria are microbes with a much simpler cell structure than many other organisms, but they are by no means simple. The more scientists look, the more they understand about how complex bacteria are. bacteria, any of a group of microscopic single-celled organisms that live in enormous numbers in almost every environment on Earth, from deep-sea vents to deep below Earth’s surface to the digestive tracts of humans.