The field of microbiology investigates life forms that are too tiny for the naked eye to detect. Any living entity that is too tiny for the naked eye is considered a "microorganism." Microbial ecology, molecular biology, immunology, industrial microbiology, and biotechnology are only few of the newer branches of microbiology that have emerged during the last 150 years. All of these emerging areas of study in microbiology have helped push the subject forward. Microbes come in different forms and sizes and may be found in the three spheres of life (Bacteria, Archaea, and Eukarya). Microorganisms, by a wide margin, are the most common form of life on Earth. Microorganisms including bacteria, archaea, protists (including protozoa and algae), fungi, helminths (parasitic worms), and viruses are all instances of biological agents. In addition, there are helminths and parasitic worms to consider. Protists are not limited to protozoa or algae, but also include bacteria and archaea. Most microbes are helpful, such as those that assist in purifying water and growing certain foods. Many of these creatures are also important to the smooth functioning of the global environment. Though certain bacteria may be harmful to the health of some plants and animals and even contribute to the development of serious diseases in humans, the vast majority are very important for the proper functioning of the ecosystem that exists on our planet.