Single-celled, animal-like, eukaryotic organisms of the kingdom Protista. Protozoans can occur wherever moisture exists. There are many parasites and commensals of plants and animals, as well as free-living species. They cause a number of diseases, such as African sleeping sickness, malaria, and dysentery. They are an economically and scientifically important group. It is thought that the organisms of the kingdom Animalia evolved from ancestors which were protozoans.