A microbe, or microorganism, is a tiny organism that is usually far too small to be seen with the naked eye. Microbes include bacteria, archaea, fungi, protozoa and some algae. Viruses are also sometimes included in this category despite their questionable status as living organisms. Sizes range from about 20 nanometers for the smallest viruses to about 500 microns for some protozoa.
Microbes provide essential biological and ecological functions. Most are beneficial or neutral, although some are harmful. One of their most important roles is breaking down dead organic matter and recycling nutrients back into the ecosystem. Many microbes form symbiotic relationships with larger organisms, aiding in their digestion and nutrient absorption, including in the intestines of humans and other animals. Microbes also play a major role in industrial processes, including in the production of foodstuffs (e.g., yogurt, cheese, beer, wine), chemicals, pharmaceuticals (e.g., antibiotics, hormones and vaccines), textiles, biofuels, and industrial enzymes.
The human body is estimated to contain roughly 39 trillion microbes, inclusive of viruses, belonging to an also estimated 10,000 species, which is comparable to the estimated 30 trillion cells. They are found throughout the body, including in the intestines, mouth, skin, nasal passages, lungs, urinary tract, vagina and saliva. The composition of these microbial communities varies widely according to the body part, and it can also vary substantially from person to person due to differences in diet, environment, genetics and health.
Among the essential functions performed by microbes in humans and other animals are helping to break down complex carbohydrates, proteins and fats in the intestines that the body cannot digest on its own and also producing some vitamins. Additional functions relate to metabolism, the immune system, maintaining the health of the intestinal barrier, detoxifying harmful substances and influencing brain function.