Protists are unicellular eukaryotic organisms that can be categorized into three main groups: animal-like protists, plant-like protists, and fungus-like protists. They were some of the first organisms on Earth and include microbes that can cause disease or act as parasites. While similar to bacteria in being microscopic and unicellular, protists differ in possessing a nucleus and generally living as individual eukaryotic cells. Examples of protists include amoebas, paramecium, algae, and water molds.