Acanthamoeba: A microscopic organism, an amoeba, found in soil, dust and fresh water (lakes, rivers, hot springs and hot tubs). Acanthamoeba also occur in brackish water and sea water as well as in heating, venting, and air conditioner units, humidifiers, and dialysis units.

Acanthamoeba 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, Acanthamoeba can enter the eye and there cause infection.

A particularly dire infection caused by Acanthamoeba called granulomatous amebic encephalitis is characterized by headache, stiff neck, nausea, vomiting, confusion, loss of balance, seizures, and coma that can progress over several weeks and end in death. Acanthamoeba infections occur more frequently in people with compromised immune systems and the chronically ill. Eye and skin infections are generally treatable while infections of the brain are almost always fatal.