Unicellular organisms fall under the main groups of bacteria, archaea, protozoa, single-celled algae and single-celled fungi. . The two general categories are prokaryotes, which lack membrane-bound nuclei and membrane-bound organelles, such as mitochondria; and eukaryotes, which are more complex, having membrane-bound organelles and nuclei.
Unicellular organisms are ubiquitous to most environments on Earth. Unicellular organisms have been found in the most extreme environments, such as arid deserts, frozen tundra, highly acidic subterranean environments, and hydrothermal vents on the ocean floor; these organisms are called extremophiles. Extremophiles are described by the environments in which they grow optimally, such as acidophilic for acidic environments and thermophilic for environments with high temperatures.
Unicellular organisms can be innocuous to humans, but they are also responsible for diseases. Malaria is caused by a unicellular parasite of genus plasmodium. It is prevalent in tropical and sub-tropical environments. There are an estimated 214 million cases of malaria worldwide each year, resulting in 438,000 deaths, 90% of which occur in Africa. The disease is transmitted from person to person by the bite of the Anopheles mosquito. Prevention includes the use of mosquito nets, pesticides and disruption of mosquito breeding grounds. There are several forms of treatment for malaria as well as ongoing development of a vaccine.