Microorganisms are diverse and include bacteria, viruses, fungi, protists, and more. Bacteria are single-celled organisms that are found everywhere and have a broad range of metabolic capabilities. Viruses, on the other hand, are not technically alive and require a host cell to replicate. Fungi, like mushrooms and yeast, are eukaryotic organisms that play important roles in decomposition and food production. Protists are a diverse group of single-celled eukaryotes that can be free-living or parasitic.

In addition to their ecological roles, microorganisms have immense importance in human health and medicine. They can both cause and cure diseases, and extensive research is conducted on microorganisms to develop new antibiotics, vaccines, and other medical treatments.