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 in humans. While similar to bacteria in being microscopic and unicellular, protists differ in having a nucleus and generally living as individual eukaryotic cells. Plant-like protists, or algae, can be autotrophs capable of photosynthesis, and fungus-like protists reproduce using spores.