What Are the Three Types of Microorganisms?

Fungi, bacteria and viruses form the three categories of microorganisms. These microorganisms, or microbes, live in all areas of the world and perform diverse duties. Some help living organisms while others act as pathogens; they come in uni-cellular or multi-cellular forms, but their small sizes make microorganisms only visible to humans through microscopic lenses.

Despite sharing the title of microorganisms, the organisms in the three subgroups vary physically and biologically. Fungi include mushrooms and mold, along with smaller single-celled organisms. The fungi group includes the largest organisms of all microbe classes. They play important ecological and biological roles, helping break down and recycle decomposing plant and animal matter in forests and wooden areas. Some fungi jeopardize the health of living organisms, generating diseases and conditions including ringworm and athlete's foot.

Fungi prove helpful too, comprising cosmetics and medicines. Bacteria occupy the second-largest group of microorganisms. They have body sizes smaller than those of fungi, and live in terrestrial and aquatic settings. These organisms, like fungi, prove beneficial and harmful to humans, plants and animals. Helpful bacteria support proper immune and digestive system functions, while harmful bacteria cause illnesses such as tuberculosis, cholera and respiratory infections. Lastly, viruses come in the smallest body sizes. Unlike bacteria and fungi, they feature simple cell designs, lacking membranes, cell walls and nuclei.