Protozoan infections are parasitic diseases caused by organisms formerly classified in the kingdom Protozoa. They are usually contracted by either an insect vector or by contact with an infected substance or surface and include organisms that are now classified in the supergroups Excavata, Amoebozoa, SAR, and Archaeplastida. Some protozoa are photoautotrophic protists. These protists include strict aerobes, and use photosystems I and II in order to carry out photosynthesis which produces oxygen. Mixotrophic protists obtain nutrients through organic and inorganic carbon compounds simultaneously.