Protothecosis is a disease found in dogs, cats, cattle, and humans caused by a type of green alga known as "Prototheca" that lacks chlorophyll. It and its close relative "Helicosporidium" are unusual in that they are actually green algae that have become parasites. The two most common species are "Prototheca wickerhamii" and "Prototheca zopfii". Both are known to cause disease in dogs, while most human cases are caused by "P. wickerhami". "Prototheca" is found worldwide in sewage and soil. Infection is rare despite high exposure, and can be related to a defective immune system. In dogs, females and Collies are most commonly affected.