A group of unicellular, usually microscopic, organisms now classified in various phyla of the kingdom Protoctista. They were formerly regarded either as a phylum of simple animals or as members of the kingdom Protista. Most feed on decomposing dead organic matter, but some are parasites, including the agents causing malaria (Plasmodium) and sleeping sickness (Trypanosoma), and a few contain chlorophyll and carry out photosynthesis, like plants.