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Acanthamoeba and Naegleria fowleri are free-living amoeba protozoan that can cause opportunistic infections in humans. Acanthamoeba causes keratitis when contaminating contact lens solutions. Disseminated skin and brain infection (encephalitis) by Acanthamoeba are seen in AIDS patients. These amoeba form cysts and cause granulomatous abscesses to form.

Naegleria fowleri

Naegleria fowleri causes primary amebic meningoencephalitis that is rapidly fatal in 4 - 5 days. The fatality rate is 95 - 100%, but a 2016 infection of a Florida teen was successfully treated (see news story below). No cysts are formed.