Examples of single-celled organisms include species in the Kingdom Protista, the Kingdom Monera and a few representatives of the Kingdom Fungi. Monera includes both conventional bacteria, called eubacteria, and a very strange group of organisms known as archeobacteria.
Rhode Island College explains that eubacteria are complex organisms, despite being single-celled. Many are important to humans, but others, such as those of the genus Streptococci, cause disease. Contrastingly, archeobacteria live in inhospitable environments, such as boiling water. Other archeobacteria live in hot water vents, located deep at the bottom of the ocean.
According to North Carolina State University, many protists cause human diseases. For example, Wikipedia explains that malaria is caused by protozoans from the genus Plasmodium, while amoebas of the genus Entamoeba cause amoebiasis. Amoebiasis does not always cause illness, but it does result in approximately 70,000 deaths annually. Malaria is even more deadly and causes more than a million deaths annually.
Discover Magazine explains that a group of single-celled organisms called xenophyophores lives in the ocean’s depths. These organisms are some of the largest single-celled organisms in the world, reaching up to 4 inches in length.
Despite including a few single-celled species, most fungi are multi-cellular, Rhode Island College states.