A microscopic organism, an amoeba of the genus Amoebae, found in soil, dust and fresh water: The free-living acanthamoeba can be found in sewage, brackish water, sea water, in heating, venting, and in air conditioner units, humidifiers, and in dialysis units.

There are two specific species, which are the trophozoite, metabolically active, and a cyst, which is dormant and stress-resistant. These species can cause the infection termed acanthamebiasis.

The acanthamoeba does not produce a flagellate stage. Its organisms are pathogens for several infections in humans and have been found in the eye, bone, brain, and respiratory tract.

They can enter the skin through a cut, wound, or through the nostrils and, once inside the body, can travel to the lungs and through the bloodstream to other parts of the body, especially to the central nervous system (the brain and spinal cord). Through improper storage, handling, and disinfection of contact lenses, acanthamoebae can enter the eye and cause an infection there.

A particularly dire infection caused by acanthamoebae, called granulomatous amebic encephalitis, is characterized by a headache, stiff neck, nausea, vomiting, confusion, loss of balance, seizures, and coma that can progress over several weeks and end in death.